If Simpsons were the only game in town, maybe the writers would work harder to make the show great. 10 years ago, the only show the Simpsons had to worry about overshadowing them was King of the Hill, and two little shows coming up called 'Futurama' and 'Family Guy'.
The Simpsons is no longer 'edgy' or 'pushing the boundaries' like it was when it first premiered, because as time went on, bigger and badder shows came along, taking what the Simpsons had done and amped it up to where now Family Guy, American Dad, and the Adult Swim shows are now 'edgy' and 'pushing the boundaries' (althought not neccesarily in a good way in a lot of instances)
400 plus episodes and a feature film is one hell of a good run. It's time to let the Simpsons rest and be comforted in the fact this was the show that brought popularity back to prime time cartoons, something that hadn't happened since The Flinstones left the air 2 and 1/2 decades previous.