To be honest though the main issue was just that it was SO successful it was able to carry on long after its time had passed, it just naturally ran out of stream losing writters and ideas ending up recycling previous ones increasingly badly.
Before that you could argue it was actually quite unique in that it really had two "primes", when it first appeared it had quite a different style(less joke heavy more drama) and more focus on Bart. Most shows I think would have been content with that, to be a fad which was hot for 2-3 years then dropped off but instead the show shifted its quite significantly and became even better.