The first three seasons or so of My Three Sons are silly. The kids are cynical, and there's a lot of slapstick. It slows down over time. Andy Griffith has an unusually strong break after Season 5 which is not just Don Knotts leaving and not just switching to color. The comedy brakes are clearly on.

The whole raft of fantasy shows were driven by outsized characters and slapstick. The rural shows were just last of those to go. Mr. Ed, Gilligan, early Bewitched, the Munsters were replaced by family stuff like Here's Lucy, Julia, Ghost and Mrs. Muir, Doris Day. They just weren't giving any priority to comedy.