Holy crap, wiki actually describes what happened to GPF!
GPF wasn't ever GREAT, but it was readable for a long time. It hasn't been for even longer.
From Wikepedia's entry on Jumping the Shark, here's why:
Sitcom characters start becoming involved on a daily basis with cloak and dagger situations (e.g., a character being accidentally recruited in a counterintelligence service, spying agency, or organized crime task force, or is simply found to be a dead ringer for an influential politician or monarch, usually from a non-existent country, often a European country, created for the episode).
Heheh, it's all right there.
From Wikepedia's entry on Jumping the Shark, here's why:
Sitcom characters start becoming involved on a daily basis with cloak and dagger situations (e.g., a character being accidentally recruited in a counterintelligence service, spying agency, or organized crime task force, or is simply found to be a dead ringer for an influential politician or monarch, usually from a non-existent country, often a European country, created for the episode).
Heheh, it's all right there.