To my mind the most famous of these 'premature' actors were McLean Stevenson (Col. Henry Blake) and Wayne Rodgers ('Trapper John' McIntyre) both of whom left M*A*S*H after it's 3rd year and when it was just getting going well.
M*A*S*H only went on another 7 years and earned the cast who stayed considerable amounts in salary and residuals as well as many awards. McLean did end up realizing how well he had killed his golden goose but Wayne has gone on to individual glory as an investor and money manager.
Wayne Rodgers doesn’t really count. He was successful in business before his acting foray, and has been successful after, as you point out.
I’d completely forgotten about Stevenson. Dying relatively early can encourage that.
In my memory, Stevenson was the only person on the Tonight Show to be able to drag out a joke across two nights.