The Simpsons, Family Guy, South Park, Modern Family, One Piece, Dragon Ball, etc. One of those shows that starts being so good but when the author runs out of ideas it starts to become a parody of itself and the quality drops. Sometimes the animation is also of worse quality despite episodes are more modern, for example the classic hand drawn Simpsons episodes have much more charm than modern Simpsons with their flawless, perfect "robotic" animation. Honestly and unironically the only long show I can think of that didn't become shit with time is My Little Pony Friendship is Magic (yes I was a bronycel), that show had good continuity, character development and kept adding new stuff so it didn't become bad. I like that they also had the dignity of giving it a proper ending instead of keeping releasing episodes, probably that's the main issue with other shows: The author wants to end them but studio says they have to produce more and more to make more cash.
