Saying that they should've called it quits is like singing to the choir at this point because it's never going to end until (and this sounds terrible) one of the major voice actors pass away, and even then, they'll probably recast. I've got my box sets of the good seasons and I think the only time that I've watched The Simpsons with keen interest in the last decade was when the Family Guy (another stale show) crossover happened. The point is: People still watch, so they'll continue with more seasons, even though they are way past the expiration date of their prime.