Joshua Alston
Sarah Ball
Ramin Setoodeh
The notion that Kathie Lee Gifford is a "quitter" is absurd. She didn't just "quit" Live with Regis - she was mercilessly and rather bizarrely persecuted by various elements of the media and she circled the wagons to protect her family. Example: The Globe paid over $100,000 to hire a prostitute to trap her husband in an affair; the first two attempts failed, the third worked. The NY DA offered to prosecute The Globe for prostitution charges, Gifford declined. Example: The Globe published graphic photos of child murder victim Jon Benet Ramsey, who was - at the time - believed to have been murdered by her mother. The Globe published these photos along side photos of Gifford's 8 year old daughter under a headline declaring that Gifford was going to "do the same thing" to her own daughter. Gifford resigned from "Live" within a week of the threatening photographs targeting her daughter.
That's not "quitting". That's recognizing that when the media is trying to kill your children, you take evasive action.
If we're going back to the '70s, McLean Stevenson, Larry Linville, & Gary Burgdorf of M*A*S*H come to mind (also the original Trapper John... not the Pernell Roberts incarnation.)
Judd Nelson appeared to be having a comeback on Brooke Shields' "Suddenly Susan" but left that one early -- and he's not been seen much since.
Shelly Long she left Cheers when it was a number one hit and with Kristie Alley came in the show did not miss a beat, however, Long's career did miss several beats. Her career never was the same again.