It's worth noting too (and I admit I probably got a little too hung up on this) that Star Trek is supposed to be about bettering ourselves and reaching our maximum potential. In the 23rd Century world of Star Trek and Starfleet, who is to say, here in the 21st, what would constitute a likely outcome of Garth's situation?
Exactly. This is fiction and you need not agree on our take.
Trying to say what would be a likely outcome in a universe that is totally different than ours (more enlightened, more humanistic) 230 years from now is all speculation.
In "Going Boldly" where Garth is back in Star Fleet, we choose to tell a positive story that has Garth returning to Starfleet, just like Picard did after leading the Borg to destroy a huge part of Starfleet at Wolf 359.
Of course Axanar has nothing to do with Garth after "Whom Gods Destroy". Axanar is the story of Garth at the height of his achievments, 21 years before TOS, when he fought a battle that saved the Federation. (which is an interpretation as well as Kirk basically says that the battle created the Federation which we know is not true due.)