Honestly to me I never viewed it as "The Simpsons Fell Off". I just looked it at as "The Simpsons just got competition". Sure I would watch it every sunday night in the 90's, by the 2000's I really only tuned in for the Holloween Treehouse of horrors special. Then I saw the famous 500th episode and it really brought back a lot of nostalgia. So I had to tune into this marathon on some episodes.
The Simpsons pretty much created the lane for Family Guy, Futurama, South Park, American Dad, etc all those Night-time Adult-Themed but kiddie looking cartoons to even come out. So once we got more "Variety" sadly the Simpsons started to get the "Fall off" Claims. But I've been watching this show since I was 9. It's amazing it's still going. Honestly I want it to end but I think people sometimes forget how legendary and iconic the show is. So I'm glad this FXX Marathon is reminding people why Simpsons = GOAT!