71dude said:
Alice - when it moved from Sunday to Wednesday in fall 1982 and the ratings tanked that should have been the end of it. The same applies to Laverne & Shirley - it was only a hit on Tuesday and died when it kicked off Thursday and Monday in 1979-80.
The Jeffersons - when they ditched the live audience for a laugh track.
One Day at a Time - five seasons was enough. Glenn Scarpelli added nothing, nor did those husbands.
Diff'rent Strokes - four seasons
Newhart overstayed by a couple of seasons.
Night Court - everyone seems to think it went downhill when Reinhold Weege left in 1989.
You could make a case that 'Alice' should have ended in 1980(when 'Flo' got her own show..and Philip Mckeon went through one of the most awkward stages of puberty ever seen in a child actor!)
Diff'rent Strokes...maybe if they'd ended it when Drummond married Maggie(end of season 6)
Newhart...I recall when I was in my teens, I used to watch both 'ALF' and 'Newhart' until they went head-to head in '88, and then I chose ALF. Except for the series finale, I never watched 'Newhart' again...and apparently, I didn't miss much.
Night Court...as mentioned in the 'secondary characters' thread, only Dan Fielding's scenes/storylines were worth watching in the last couple of seasons. Harry Anderson went from being a loveable geek to a boring wuss as he branched out into other acting roles, and that turned Judge Stone into a bore, as well. Christine marrying the idiot cop, splitting up, and then NOT continuing a relationship with Harry was another turn-off. Oh, and Gilbert Gottfried turned up in the final season....and I haven't even mentioned the half-baked final episode.
I'll add one of my own...'Empty Nest', post-1993. The decline had already started the year before, when Kristy McNichol left due to herpersonal problems. They hired someone to play the previously-mentioned but unseen third daughter, Emily, and she did OK...until, in the fall of '93, they revamped the show. Emily was gone, Harry was no longer a pediatrician, but worked in a free clinic with Marsha Warfield(!), and the writers ran out of ideas and turned Carol into a Murphy Brown-ish single mom(after she dumped her boyfriend, played by Paul Provenza). And, out of desperation, suddenly Sophia Petrillo was back in the neighborhood, and constantly popping up.