The Simpsons were pushing the envelope when the show came out, doing things that were considered almost radical at the time. But they are just stuck in time, following their formula while subsequent animated shows (e.g. South Park) have gone much farther than the Simpsons ever did.
The show often also feels like the writers are just rehashing old ideas, and there seem to be a few archetypes of stories that just keep popping up over and over again: "Homer is an incompetent idiot, but comes through for his family when needed", "Lisa feels alienated, because she's much smarter than the rest of the family", "Bart is a bad student and gets into trouble", "Marge is bored with her existence as a housewife and tries to break out of the monotony". At some point you should just admit that you have done everything you would with a concept and call it a day instead of dragging things out.
That's true of a lot of long-running shows. Even if they don't become worse in the sense that individual episodes are worse taken in isolation, they're often just repetitious and, as you say, stuck in time.
You see this elsewhere too. In comic strips, for example, Dilbert is mostly stuck in some 90s version of cubicle life at a big company like Pac Bell. Don't really reaad it any longer but even when nothing is wrong with a given strip I've probably seen some version of it 10 times before.
The show often also feels like the writers are just rehashing old ideas, and there seem to be a few archetypes of stories that just keep popping up over and over again: "Homer is an incompetent idiot, but comes through for his family when needed", "Lisa feels alienated, because she's much smarter than the rest of the family", "Bart is a bad student and gets into trouble", "Marge is bored with her existence as a housewife and tries to break out of the monotony". At some point you should just admit that you have done everything you would with a concept and call it a day instead of dragging things out.