I'm not sure I entirely buy the logic in that. It might explain why it may be having trouble findiong new viewership in people not routinely exposed to the Bradeys, Huxtables, Tanners, Seevers, ect, but not why those of us that still can get the parody think it fell off somewhere.
I still contend the problem is overexposure, and not just in that after 400 episodes, it's hard to stay fresh. Watching the Christmas episode this year I did a double take as the photos they used to progress to the future showed lisa had brought home girls as significate others. That was a story I wouldn't mind seeing, and not for obvious reasons. It just feels like we've done everything and then some with Bart and Lisa as children and it has long passed the time to move on to their teen years. Ample parody fodder there in sit-coms and dramas. Sadly, they've been stuck in their ages, characters and jokes, and the rest of the cast has largely been the same. Yeah, we sober up Barney for a few years and hook up Skinner and Krabapple for a few, but little sticks, Mod Flanders' death being the only one I can think of. Comedy needs to stay fresh, and the Simpsons' is up there with OJ jokes.