Your complaint is really not reasonable when you consider the logic behind it. Wild Growth was nerfed because it is in the basic set, and it will continue to be in Druid's identity forever until nerfed/removed, the fact that it still sees play after the nerf is proof it wasn't overnerfed. It made sense to nerf it because if they didn't it would continue to be in almost every Druid deck for all time.

On the flip side, Overgrowth is NOT basic/classic, it only exists for 2 years. It makes perfect sense to have high powered cards that define a class within expansions, because at worst they exist for 2 years naturally, and once they're gone they are no longer a constant.