Well, MASH simply outlived its ability to come up with believable story lines. Most series are like that. After about five years, virtually every sitcom or drama becomes stale. MASH was no different. It always was a little preachy, but the writing overcame the preachiness. When Wayne Rogers left, the series went downhill fast.
The show lasted three times as long as the Korean War.
It lasted 4 years. After ‘74 it went downhill and increased it’s speed as time when on. Henry Morgan and Alan Alda were all that was left after Henry, Trap, Frank, and Radar left.