Ah, the old debate of "When did the Simpsons start to suck"...
I'd say eleventh season (1999). The Mel Gibson Episode was great as was the finale with the fake behind the scenes doc, but still that season had the episode with Neds Wife dying (wich failed to be either touching or funny), the one with Barney sobering up (another attempt to suddenly have plot developments that kind of go nowhere), and the infamous "Saddlesore Galactica", aka "the one with the other horse and the evil jockeys" (rumored to be the writers protest against the cancellation of Futurama, a reaction to bitchy fans complaining about repeating storylines or...well, just a bad episode) that is probably still the worst Simpsons Episode of all time.
While there where enough later Seasons that were much better than that one, the eleventh season was still a very VERY hard drop in Quality of what was until then an almost consistently great show (well, if you want to be precise, some episodes of the tenth season showed signs of fatigue, but i'd still say it was overall still a good season)
But yeah, as you might have guessed, i think that the Simpsons should've been cancelled long ago. I don't blame the voice actors for it, and they should leave the obviously sinking ship since even if they get what they want, it will only get worse from here.