Well, if the attraction is great, it doesn't matter if the source material is outdated or not known in my opinion. Look at Splash Mountain. How much of the public has seen Song of the South? Not much but the ride is still fun and amazing and popular! Same, I think, counted for the likes of Jaws, BTTF and ET. If the ride is well made, it will stay popular. Universal, in my opinion, jumps the gun on judging what's popular.
REGARDLESS, even if The Simpsons show was done, but the ride was well made and amazing, it wouldn't matter that the show was done. The ride would hypothetically stay popular! I give The Simpsons Ride 5 more years MAX. If it lasts more, I will be extremely surprised