Still, the point is people tend not to think about declines once the show/person/whatever stops. Sure, we can complain about the Simpsons decline all we want, because it is happening to us right now, but do you think people will honestly look back on the show once its ended and say "What a no good show. Sure it started good, but became terrible"? I would like to bring up that Tom and Jerry, a classic beloved by nearly everyone on this forum, had to endure decades of mediorcy (And not just spin-offs either, but rather the shorts themselves), and even the Looney Tunes themselves faced a similar situation. Even Happy Days, the show that coined the pharse "Jumping the Shark", is still remembered as a fun little show today. Do people know that there was a decline? Of course. Does that change anyones opinion of these series once they end? Not really, because once its all said and done, people tend to judge a series based on its influence and its best work. The Simpsons has created some of the best episodes, characters, etc. of any show ever created, and while they are currently being misused, the future will judge them for what they did right.
In short, I dont feel that thier legacy is in any danger, they will always be considered a classic and a milestone in both animation and television in general. That being said, yes, these current seasons are horrible, and as a Simpsons fan myself, it is rather unfortunate to see them like this.