I think people just get tired of shows - especially one like The Simpsons, which has aired for so many years. I stopped watching it regularly about a decade ago. But anytime I do watch a new episode, I am still highly entertained, so IMO - the show has not 'jumped the shark.'
My problem is that I have tired of the TV sitcom genre in general. As great as 30 Rock was, for example, I rarely got around to watching it. There are just too many great basic and premium cable series (mostly dramas) to watch these days, and just so many hours in the day. It's a great 'problem' to have.