My go to reference for this is Search Party. The first season is the most engaging TV I've ever seen, the second season is still pretty damn good, but then the show got picked up by HBO and became total shit for 2 seasons. Like god I fucking hated season 4 of that show.
As far as movies go, I gotta go with Shrek the Third. 1,2 & 4 are all great, but what the fuck was 3?
I think a TV show has a set story in mind, and roughly how that will play out over 3 seasons or so, then it gets wildly popular and the network says "yeah we're gonna need 9 seasons out of this IP" and then the writers have to figure out how to add some rake kickflips and unnecessary characters when they really don't have anything more to say with them.