Some of those shows were quite popular. Of course, popular does not always equal quality
I think a better description would be overrated, with the addendum that most of these show still had strong ratings
"Cheers" post season 4, "All in the Family" post season 5 (when Mike and Gloria moved out, the conflicts were not as strong"
"The Jeffersons" maybe post season 5 (At that point, the performances seemed "phoned in"),
"MASH" post Frank Burns(some seasons were still strong, but there was no effective adversary to the better natured Hawkeye,
Margaret became more sympathetic, and frankly, Hawkeye's self-righteousness became intolerable)
"After MASH" not really a bad show, but just boring ans a shadow of the original
"Hill Street Blue" was never as good, once Michael Conrad passed.
"Two and a Half Men" - not necessarily because of Charlie Sheens's departure (Ashton Kutcher was agood casting decision,
ratings wise and as a more ethical character), but because Alan became creepier and
Amber Tamblyn was written as cartoonish.
"LA Law" post Harry Hamlin (whether his character was likable or not, the show's dynamics didn't work anymore)
"Friends" the entire run, IMHO, the characters were just too smug
"Happy Days" post Ron Howard
"Big Bang Theory" last 2-3 seasons" (I never liked those Raj Mystery episodes. I would just change the channel)
More than enough has been posted about the decline of "Dallas" and "Dynasty"
"Melrose Place" post Courtney Thorne Smith and Grant Show
"Beverly Hills 90210" last 2-3 seasons
"Buffy the Vampire Slayer" season 7 - less than satisfactory final season, did not like the finale
"The Sopranos" first two seasons were great, season 3 not, season 4 was elevated by Steve Buscemi, later eps/seasons not as good, did not like the finale)
"Thirtysomething" self-absorbed, unlikable characters. Depressing
"Designing Women" post Delta Burke (although I think Julia Duffy was a good casting choice, the scripts failed, not her)
Shows (scripted) that were really bad - "Mama's Family" (Not funny at all IMHO, just mean spirited, really over the top trailer)
"Joanie loves Chachi" (horribly written)
Reality shows - almost all of them