Yeah it would be difficult to keep finding jokes after about 15 or 16 seasons...they don't have as good stories as they used to...they've ran out of small family style stories as they've pretty much gone everywhere they could...its much more joke driven which is still funny but not as good...the fact that the older ones had good down to earth stories was the reason why I prefer the Simpsons to other comedy shows like Family Guy (which rocks but while it does have good replay value it doesn't come close to the Simpsons)...

What annoys me is that Fox cancelled Family Guy and Futurama which were both great shows and were both relatively young with plenty of new ground to explore, and yet they gave the Simpsons extra series despite the fact that that show has kinda been done to death...well at least they're bringing Family Guy back, but theres still no plans to bring back Futurama as far as I know...