I really dislike this speech, because this is one time Mike is dead end wrong. If Walt had "shut up, known his place", he'd be dead because Gus intended to kill him ever since the Jesse incident.
But as far as "no going back", it depends on what you mean. If you mean morally, walt killing mike isn't even a particularly bad action. Mike was a thug and hitman, of all the suffering and pain Walt caused, taking out of someone like mike because of spite and security reasons doesn't even hit the top 10 and I think anyone who argues it does is just letting their parasocial attachment to the characters bias them from a rational position.
Butif you meant Walt being fully committed to a life of crime, that's points been long past, baby.
I would argue that's because Mike isn't referring to the state of their relationship in Season 4, but pre-Abiquiu in Season 3. Even at this stage, Walter is flirting with the idea of killing Gus which was largely driven by worry over what happened at the end of their contract.
While Gus at this stage already had reason to be annoyed with the replacement of Gale with Jesse (partially an insurance policy on the part of Walter, partially a method to silence Jesse), there isn't yet an open hostility on the part of Gus but Walter has absolutely explored the possibility of killing him both to reaffirm the security at the end of the contract and to a lesser extent due to greed highlighted in Kaefkaesque (which, ironically, helps reaffirm to Walter how dangerous Gus is by seeing the operation's scale and emphasising his danger).
At the same time, I do agree that the argument is flimsy. There was good reason for Walter to be skeptical of how much of a danger Gus represented towards him and a justified reason to think that Walter would not long-term be allowed to live. The speech from Mike is likely shoe-horned in as a way to try and re-emphasise to the audience members who haven't yet comprehend that Walter, ultimately, is not a good person and is doing this out of his own selfishness rather than a 'need' to do it.
I agree that there would have been more reasonable things for Mike to point out and while ultimately Mike is correct that their predicament ultimately boils down to Walter's actions, there probably were better things that could have been written into the scene.
EDIT:
This scene and the season at large ruined Mike as a character, they cut him up because they needed someone to push back on Walt even if it didn't make any sense. Mike, who supposedly very close to Jessie at this point is telling Walt he 'ruined everything' by not letting Jessie get killed.
Similarly, while I do agree that they needed somebody to call out Walter and it's probably why it was written in, I think it stretches back further than Half Measure. Mike had a very justifiable reason to butt heads with Walter in the season which very much was in both of their characters, but I do agree the 'climactic' scene could have been better written on Mike's behalf as there were many better things which could have been raised to highlight how their situation traces back to Walter.