We all know the go to explanation, Linville was offered a two year renewal but decided to leave because he couldn't do anything more with the Frank Burns character, but that explanation has the crackle of confederate money. Beginning with season 4 the show was moving in a decidedly different direction. Gone were two characters from the movie who were happy to participate in the sexual antics that so punctuated the film and the first three seasons. Potter and BJ were very married and took the commitment seriously. Beginning with season 5 Hot Lips is engaged, so she and Frank are no longer involved. The series had clearly moved on so why would the production company be interested in retaining Linville? And why if they were willing to retain him would Linville walk away with no other jobs forthcoming? Linville was recently divorced, we can assume he had alimony and child support to deal with plus he was getting married again. Linville was no star, he was a regularly working character actor with a paycheck, doesn't make sense he would walk away from that. This seems more like a production company decision, the show had Alda who we can assume was the highest paid actor, plus Morgan who must have come at a premium, plus Swit who as the female lead in her sixth season was expensive, Burghoff an actor with a bit of an ego (to say the least) who again in his sixth season was probably not cheap, Farrell (a relative unknown at the time but he was a regular) and Farr and Christopher who had gained regular status. Sitcoms of that era generally worked with 4-7 regulars so MASH with Linville would have been 8. These things almost always come down to money, isn't it more likely that the production company as a courtesy allowed Linville to indicate he was not renewing rather than announcing the character was being written out. Basically they let him go or made him an offer he had to refuse.

I actually did not know any of that!

How much did McLean Stevenson's abrupt departure play a role in changing the overall group dynamics on MASH? What I mean is that had McLean stayed, then would it have made much more narrative sense for Larry Linville to stay?

When Sherman Potter replaced Henry Blake, now Margaret Houlihan (https://hero.fandom.com/wiki/Margaret_Houlihan) had a commanding officer she could respect and even look up to as a father figure. This in return, meant that she no longer had to conspire regularly with Frank Burns to undermine or go over the commanding officer's head as they did with Blake.

Frank was now deprived of his co-antagonist and in essence, became the show's sole "villain" (https://villains.fandom.com/wiki/Frank_Burns_(television)).

Interesting points, but all speculative.

I believe that Linville was tired of the character and let's face it, the character had been written into a corner.

Even if Linville had stayed, he probably would have lasted maybe one season, tops.

It was time for him to move on.

Sitcoms of that era generally worked with 4-7 regulars so MASH with Linville would have been 8.If that were true David Ogden Stiers wouldn't have been hired as a full time replacement. M*A*S*H remained at 8 regulars until Radar left.

Frank would not fit into the Mash Melodrama the show became.

I can see why Larry was sick of it. They turned Frank into a complete punching bag by his final season with no redeeming qualities. the character had become a over the top cartoon character. the writers blamed themselves because Larry was so good at his job they could not resist portraying Frank like that. Once they split Hot lips and Frank up and he was on his own with no friends the writing was on the wall.

If that were true David Ogden Stiers wouldn't have been hired as a full time replacement. M*A*S*H remained at 8 regulars until Radar left.

Stiers was a relative unknown just starting with the series, Linville was a regular who would be returning for his sixth season so the difference in salary would have been significant. In addition, just the fact that his contract was up for renewal on this very successful series would suggest he was looking for a considerable salary upgrade.

Frank would not fit into the Mash Melodrama the show became.

I agree. Much in the same way that Colonel Flagg made his last appearance in season 7, because that character fit better in the first five years of the show.

The show evolved and Frank Burns would never do so.

When asked about this, Linville himself said, "What do you want him to do, become Alan Alda?"

I think that pretty much sums it up.