It totally made sense at the beginning of The Simpsons to have ~8 people doing dozen of voices each.
They probably could have gotten around paying the core group as much if they had split the voice into more actors so they didn't have such power to demand hundreds of thousands an episode. Greed though is how Hollywood works.
They should have done this sooner but probably thought it wouldn't look bad by waiting a decade so people don't have a recent comparison.