>>52360667Gen 5's issues stem mainly from the ridiculous general power creep + the unleashing of weather. So many people back in the Gen 3/4 days thought that Swift Swim/Chlorophyll were generic abilities that weren't that helpful that made up the majority of abilities at the time, but the moment you add in a weather setting ability, they suddenly became amazing. That and things like gems, Volcarona, Keldeo, and Sand Rush made Gen 5's powerlevel explode.
But that's the thing with "generalist" abilities. Very few Pokemon had signature abilities/moves, so there almost always was an alternative. Power levels rose in tandem because if Kingdra could make great use of Swift Swim and brought Rain to the forefront, it'd also drag along Pokemon like Rain Dish Tentacruel, Dry Skin Toxicroak, and Storm Drain Gastrodon. Everything was interlinked because of the design philosophy at the time.
Just as an aside, look at how many Pokemon in Gen 8 OU had signature moves/abilities.