I can think of several TV shows that were destroyed when a star left: Good Times (they should have promised John Amos all new writers and doubled his salary to stay), Happy Days (not that it was that good before Richie left), Bonanza (already aged, after Hoss died it just shriveled), Golden Girls, Designing Women, etc…
Some retained quality after a *supporting *character from an ensemble left. (Barney Miller survived the death of Jack Soo and departure of Abe Vigoda okay and very possibly would even have survived Hal Linden with a name change since workplace comedies have a lot mroe lee way. Sanford & Son handled Redd Foxx’s temporary walkout fairly well by having Grady move in for some of the show’s funniest episodes (“No orggies!”) but probably couldn’t have carried on indefinitely (though I think Whitman Mayo’s surprise popularity convinced Redd to make a deal and come back).
However, did any shows in your opinion ever improve when a major star departed, however?
The closest I can think of is CHEERS. I won’t say that Shelley Long leaving improved it in terms of acting and plot and all (Diane was a pain-in-the-ass character at times and I know the stories that the cast hated Long but she was a great foil for Sam) but bringing in Rebekah added a transfusion to a show that had done about as much as it could without turning into a “Sam and Diane get married and have a baby in an elevator” type show.
“Cheers” was the first thing that came to my mind. Yeah, the whole Sam/Diane thing was cute, but I found Shelley Long irritating as hell. I much preferred Kirstie Alley as Rebecca.
Yeah, Law and Order is definitely the template for a show improving after the departure of major characters. Of the “classic” line-up, only Chris Noth (Mike Logan) and Steven Hill (Adam Schiff) appeared in the first two seasons. Special Victims Unit also improved when Ice-T (Fin Tutuola) replaced Michelle Hurd (Monique Jeffries).
MASH got incrementally better when Wayne Rogers, McLean Stevenson and Larry Linville left. Oddly, Gary Burghoff’s departure hurt them some.
The Avengers improved when Ian Hendry, the original star of the series, left and Patrick McNee, the sidekick, took his place. When Dianan Rigg replaced Honor Blackman, they became a true hit.
Star Trek: The Next Generation improved after Denise Crosby left as a regular. And I actually liked her Sela character (Tasha Yar’s half-Romulan daughter) better than I liked Tasha.
The best example that I can think of is A Different World. After Lisa Bonet left (and Debbie Allen came on board as director), the show improved dramatically. The first season is absolutely nothing like the other seasons, as a result.
On the same note, Elisabeth Röhm’s departure from the original version was a terrific thing for the show. She was wooden and unwatchable in that role. The only other thing I’ve ever seen her in, Miss Congeniality 2, she went quite a ways towards making that movie a crapfest as well.
I don’t know if you’d call her a star of the show, but the departure of Veronica (Robin Tunney) at the beginning of Season 2 was the best thing that happened to Prison Break.
I think The Adventures of Superman improved when Noel Neill replaced Phyllis Coates. Neill’s Lois Lane was more fun.
If ratings mean anything, Toma improved when Tony Musante left and was replaced by Robert Blake (and the show renamed to Baretta).
While I disagree strongly, most people think Mission: Impossible improved when Peter Graves joined the cast (so much so that few people realize that Stephen Hill originated the role of the IMF leader).
One can argue, but most people think the best Doctor Who is someone other than William Hartnell; there are nine different points where you can say the show improved by a change in the lead.
Similarly, Blake’s 7 survived and thrived after the loss of Blake as well as other members of the seven during the course of the show.
I never liked Diane Muldaur as the ship’s doctor. For whatever sordid reasons Gates McFadden was supposed to have left for, I was glad when she returned.
As did all writers of Picard/Crusher romance fiction
Mission: Impossible was probably made better when Steven Hill left and Peter Graves replaced him.
I think that Star Trek Voyager would have been significantly improved if they had replaced Kate Mulgrew. Every time I watched the show, she just really came across to me as a total shrew.