1999 would have been a great time to end it. There was nothing left to do with the characters. They had done episodes where Marge wants to do something different, like the one where she joins the police force. They had done all sorts of episodes about Homer doing crazy stuff, like enrolling in clown college or the one where he gets even fatter or when he became a bodybuilder. Tons of episodes about Bart’s shenanigans with teachers, Sideshow Bob, Sideshow Mel, bullies, etc. They did a wild episode where Homer crossed over into the 3D world. They did an episode set 15 years in the future where Lisa gets married and we see Bart as a loafer still underachieving. Every character had been explored in hilarious ways. Also, that was the time the new series Futurama was coming in. Instead of letting it end with dignity they decided to keep it going and milk it for all it’s worth. I catch glimpses of it now and then and wonder why on earth it’s still on the air.