You can draw a line where the movie came out that splits the run of the show. Before it, an integral part of the pop-culture landscape. After it, oh yeah, guess they’re still making that.
The movie itself is… pretty good. It’s got a really high gag hit rate, and its plot about the cavalier attitude of Springfield residents towards the environment is, sadly enough, not dated at all. Both the big-names-as-themselves cameos (Green Day and Tom Hanks) remain superstars. The animation looks terrific and the bit with Bart’s penis is one of the single greatest gags in any movie ever. It feels bigger and edgier enough that it does feel like an event, rather than just a long episode – Homer flipping the bird, Bart passing out drunk, Marge saying “goddamn”, and at one point the Springfield baying mob is literally preparing to hang a baby. It even gave us one meme, possibly the last great meme to come out of The Simpsons: “This is the worst day of my life!” “The worst day of your life so far."
But the pretty-goodness of it all is quite overpowering; the way it kind of comes and goes yet everything remains the same. Springfield is destroyed, but other than a few references in season 19 to rebuilding it and the occasional subsequent appearance of Plopper the pig, everything basically immediately returns to normal. There are no lasting character changes, no developments of any kind really, just a comedy pig. It feels pretty inconsequential really, after years of anticipation.
It suddenly just felt very inessential. Whether consciously or not, that first generation of fans were shown that this thing they’d loved for a decade and a half didn’t necessarily love them back. While their own lives moved forward and changed, The Simpsons was staying right where it was, content to keep going, largely unchanging, forever.