It's worth noting that Sheare has spearheaded negotiations and threatened to leave before. A few years ago the cast agreed to lower their pay from $400k per episode to $300k.
He is also one of the only cast on an ongoing tv show I've ever heard of criticizing his own show publicly.
I don't blame him if he is tired of working on the show. It can't just be Internet pundits who are tired of the falling Simpsons quality.
More on Wikipedia http://en.m.wikipedia.org/wiki/Harry_Shearer#The_Simpsons