I completely disagree with gym leaders being high level off the bat. People already struggle with the level grind as it is, and most good leveling areas require 16 badges to access as it is. Even if you changes these areas to be open, it still wouldn't work.
The completely wild grind would be a huge turn-off for many people, because there would be absolutely zero "progress markers" that would encourage people to continue with their journey. The game, for most people, would consist of walking left, right, and then choosing an attack to O-KO a wild pokemon. Then, once they hit level 80 (Something that takes quite a bit of dedication, especially if a person has just jumped into the game) MAYBE challenge and beat/lost to a gym leader.
The gym leaders are there to give players a reason to progress. The ELITE 4 is there to be a more serious challenge, not the gym leaders. The gym leaders are designed as progressive, encouraging obstacles.
If you want to make them rebattle-able later (sort of like the lategame areas of black/white), then they could be uber awesome fighters that give moderate (but not necessary) rewards. Even then, this would appease the more PvP-based players instead of the Collection or Social-based ones.