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It won't mean an end to the series. The Simpsons is scheduled to go on for at least 19 seasons, and it is currently in its 18th. Fox is unlikely to cancel the show that made them, so I think we'll see it stick around for a while.
That is unless the show gets really horrible. I don't think that's very likely, though, as the episodes currently airing are around 7/10 in terms of sitcom quality, in my opinion.
Hopefully some of the classic writers of the show sign back on after the movie. The Simpsons really evolved through their writing, and their comedic style being filtered through the characters. That's what's let the show down most of all...the writers just have a different comedic make-up to the writers of old. They'd be better off writing their own animated sitcom, while writers who know the style that made The Simpsons great remain writing for "America's Favourite Family".