When a show lasts as long as The Simpsons
, it's quite possible that a viewer's tastes change and they no longer relate to it as much anymore. It doesn't mean the show itself has "lost its way" or gotten significantly worse, but just that the viewer has moved on and just isn't interested anymore.
What's funny and hip to a viewer at 13, isn't always the same at 34.
Or else the show actually could
just become stale.