We have always had those old rumors that the show as going to end after that particular season.....I remember an old rumor that the Simpsons was going to end after season 10, and another one that the show could end after season 20.
The show gotta end eventually...as it depends on how long the main & other regular voice actors for the Simpsons family characters and the recurring characters can keep going as most of them are already in their mid 50's, and a few in their 60's and Harry Shearer is 75. We already lost 3 or 4 voice actors/actresses that already passed away.
I give the show at least another 4 years (to season 35) before the voice actors/actresses decides that they have done long enough for the show.
I don't think the show will get to season 40.