I'm a lapsed Simpsons fan but I find it hard to pin point when did the show really suck or what should have been the proper solution to addressing the decline.
The only definite viewpoint I have is that I believe the Simpsons Movie came out too late. In the video, the guy talks about how early Simpsons tried to be clever with its storytelling/jokes and it was true. The Simpsons Movie clearly wanted to do the same thing, but when the current show was surviving off of using celebrities, the movie was caught in the crossfire and couldn't deliver anything that would appeal to lapsed fans who wanted a story, and the more modern "pop culture" one.
The result was just a "passable" movie but not one anyone talks about anymore. Had they made a movie in 1999 like South Park did, I think they could have put out something more memorable.