Natures are arguably the most important part of a pokemon, along with abilities. EVs are mostly for customization purposes, whereas nature and ability are for optimization purposes. An example would be unnerve Tyranitar with an adamant nature. Adamant allows for the 10% boost to the stat, while unnerve allows tyranitar to be used on a team that would otherwise be negatively effected by sand. From here on forward, I will use Tyranitar as an example for many of the sections of my post. I intend to do so because tyranitar not only serves as a prime example of all of he topics below, but also because of its mainstream use in the OU tier.
1) Concerns with natures in general:
Natures are extremely important within the context of competitive battling. If you do not have a proper nature, even with everything else being correct, you have a useless pokemon depending just how bad that nature is. Not every pokemon is as versatile as something like Tyranitar. Tyranitar has the freedom to use many natures due to its great movepool and stat distribution. However even with a pokemon as wonderful as tyranitar, you CAN NOT use a neutral nature effectively as it is not optimal. You will always want to boost a stat and lower another depending upon your moveset and EV spread alongside your ability.
If you have a docile tyranitar (level 100) with EV investment of 252 hp 252 attack, and you are fighting a rapid spinning starmie, with a timid nature and EV investments of 252 hp and 252 speed, assuming all 31 in IVs for the sake of consistency, the tyranitar does 94.4-111.7% damage with crunch to that starmie. It has a 68.8% chance to OHKO the opposing starmie with a neutral nature.
If you take the same tyranitar, but make it adamant nature, it then does 104.3%-122.8% damage to the starmie, assuring the OHKO.
If you take a different tyranitar, with 0 ivs in attack, but with an adamant nature you then proceed to do 95-112.9% damage to the same starmie with crunch.
If you notice you do more damage with a proper nature and 0 ivs in attack, than you do with an epic attack tyranitar with a nature that is neutral to its attack.
The above example was used because tyranitar is a premier counter to starmie and is recognized as such especially in sand.
This proves indisputably that if we are not given the chance to change ability from neutral nature as it is currently, then it effectively and literally ruins any pokemon that currently exists in reference to their primary stats even if they are literally perfect 31s.
2) Concerns with 28+ Shinies:
The next question follows necessarily. "If natures are that crucial, wouldn't it just make the 28+ shinies unstoppable and unfair?" Now before I answer this, I am aware that I am speaking from the standpoint of someone who is very well off within the game. I understand the concern and it is a valid one, however, unlike the first segment, this one is not as directly detrimental to the outcome of a match.
As addressed in the first post, the nature matters more than IVs in primary stats with any significant base number behind it. How does this translate over to 28+ shinies? Well there are a few points we need to address before we get started!
A) The sweeper speed dilemma
If two pokemon such as Espeon and Gengar go head to head, what determines the victory? The Espeon has a S.T.A.B. psychic type move that is super effective vs Gengar due to it being poison type. The Gengar has a S.T.A.B. shadow ball that is super effective against Espeon. Both can OHKO each other with these respective moves. They both have the same base 110 speed, so nature is the thing that comes into play here. Gengar typically runs modest nature to handle other pokemon, whereas Espeon always runs timid. This would mean that Espeon wins this exchange 100% of the time, barring focus sash. The Gengar in this instance loses out. Now let us supposed that this Gengar is shiny and max speed, since this was a common it is fair enough to expect. This trainer knows this matchup would normally swing towards Espeon, and also knows that since Eevee is HR, that a max speed Espeon with other useable IVs are hard to get. This trainer then opts to run Timid on that Gengar, to fight espeon. This would result in a speed tie and an equal chance at moving first, or in most cases, the Gengar will then win and OHKO the Espeon.
That being said, if we are not allowed to adjust natures of old pokemon, we are actually at a disadvantage to use old IV shinies in many cases since the other player would not expect a max speed, 28+ special attack, shiny Gengar with new ivs to be in the hands of most players.
The same could be said of this Espeon. Players up until this point have had years to collect battling pokemon. If they were unable to change the nature of their Espeon that they have used in battle for years up until this point, they will always lose out vs new Gengars that are lucky enough to get timid, no matter the IVs. It essentially shifts the luck factor without removing it, undercutting the argumentation of "Catching a pokemon with good ivs should not determine the outcome of a match." This would make all old pokemon obsolete and as such make everyone start over with every pokemon they have. This does not remove the current problem but just shifts back the problem a few years.
This also gives further issues to pokemon with moves such as dragon dance. Most of the dragon dancing crew are very rare pokemon, and as such are hard to find with good overall ivs. With speed being a crucial number in calculation of dragon dance, you need to be able to hit certain thresholds in your stats. Sometimes a pokemon cannot hit a certain threshold without a 31. Prior to the EV update, a +1 dragon dance Tyranitar needed 31 ivs in speed, in order to outrun a max speed Salamence after that one dragon dance. That one IV in speed was the crucial factor where it determined whether or not a Tyranitar would be able to boost vs a Salamence. This is huge mentally as well for the Salamence player in choosing how they battle and what moves they use or if they should switch.
B)The stall/tank dilemma
Defensive oriented pokemon benefit more from generally lower IV pools and lack of natures. Defensive pokemon always have to prioritize one stat over another due to the way natures work. This means that some counters actually work or do not work, dependent upon the nature and investment of the pokemon. An example of this would be close combat from a strong fighting type vs a Snorlax with defense investment/nature not getting the OHKO, whereas if they invested specially rather than physically, it would OHKO.
Some pokemon have the ability to go defensive in either stat, such as porygon2. With eviolite, it gets monstrously high defenses. It has base 90 and 95 defenses respectively. Both are sizeable and it has a reliable recovery move. Right now, Porygon2 can just invest evently in the stats and wall just about everything because we have no opportunity cost due to natures right now. The same can be said for dusclops. This means you can play with those pokemon the same way, regardless of what you are fighting. That being said one of the major ways to counter these pokemon, is within the use of natures. Dusclops and Porygon2 both end up opting for one of the defenses and using a nature to support that defense. This means if an offensive threat that opted for the opposing offense + nature fights that pokemon, they now have an advantage they otherwise wouldn't have.
That means if you force everyone to get new pokemon, tanks will have the advantage for years since finding offensive IVs to match up with the proper natures takes much longer than versatile defensive types and also creates a higher price spike in offensive type pokemon due to the inaccessibility of these pokemon to the common player.
TL;DR Tanks will be cheaper due to having a higher versatility with natures than offensive pokemon typically do, creating imbalance within the competitive community.
C) Conclusion
28+ IV shinies are not the problem, and will not be one statistically. The amount of benefit IVs give mathematically are low, and only matter in very rare cases such as speed ties which are already rare occurrences as is. Player skill in a full battle system which includes custom set making, choice of nature, and custom ev spreads for the niche use of that pokemon, are what will determine the outcome of a battle more often than not.
Concerns for longevity of the game:
28+ shinies, as you will notice, increased shortly after the implementation of 20+ IV shinies. The prices of non-shiny epics have not changed. This means that the accumulation of 28+ shinies will become harder and harder over the years, even among past commons and event commons. (I paid 45m a few days ago for a max speed S gengar with old ivs, whereas over a year ago they were 10-15m.)
With the increased value of old iv shinies forthcoming and continuing, they will become mantles or badges of status to players. This means they will be decreasingly traded in the coming months and years. Players will be more inclined to go inactive and keep their pokemon in case they ever come back, rather than sell them for pokemoney or trade for new iv pokemon. Over the course of a few years they will cycle out more and more. Many inactive accounts are worth a ton right now with all their old S common battlers that will eventually just get deleted due to inactivity or bans in some cases.
The "problem" will fix itself given a few years by nature of economic flows. Rare shiny stuff for the most part will become less and less widespread. It wont be the norm and isn't the norm even now. It is far easier to get epic IV pokemon than their shiny counterparts for everything besides commons so it really isn't an issue to begin with. It is also cheaper.
TL;DR This means that the amount of old iv shinies in use, will drop with time, and the longer 20+ ivs are implemented, we will experience crowding out in the market place, causing the standard to become 20+. Once this crowding out occurs, the downfall of 28+ begins since no one will trade them unless it is for something extremely rare. (This is already happening for R+ shinies.) I just had the displeasure of spending 130m on a max speed S arcanine yesterday because of how rare they are now. Just a year ago they were only 35-40m.
Concerns for player approval and interaction:
If we are not given the ability to change nature, it ruins all of our pokemon for the people who have been here for years. It also requires us to essentially start over having nothing to show for it in terms of the competitive scene. We would all have to go re-hunt for literally every pokemon we own since neutral natures are never used for anything.
Most of us enjoy our time here very much, and we would like to move forward with the game in a way that we can all learn from one another. Battles are a way to match wits and learn about our own personal play styles. As new players join, they can spend some time getting a team, and learn how things work. After they find some favorites or have a few goals in mind, they can then spend a lot of time perfecting their team and working on those goals like many of us have done for years. The reality of always finding something better and having a goal to achieve is what keeps players working on MMOs. Everyone wants to achieve the dream "to be the very best, like no one ever was."
P.S.
I got tired halfway through this monster, so please be charitable to potential misunderstandings. I will supply responses upon request or clarifications where needed.
TL;DR
Let us change natures and abilities but make it something to work towards, just like hunting is working towards finding that special pokemon.
But I get your point, I also have your problem, sitting with 600m and nothing to buy...