The reason not to do this is that you're very likely to provoke a defensive response and have the other person dig in their heels. A gentler approach is more likely to get results, less likely to create tension in the relationship, and sets the tone of future decision making as adversarial.
If that doesn't work you can try firmer approaches, and you can of course always quit or use the threat of quitting, but 99 times out of 100 leading with that will lead to worse outcomes.
You don't give up any leverage by first trying to resolve conflicts without threats.
This is where I disagree. The magnitude of response should be determined by the likelihood of achieving the desired outcome. Sometimes that requires drawing a really firm line, but usually it does not. And I'd argue that it virtually never involves drawing that line as your first strategy.
Like I said, you should absolutely be willing to make that threat, and to walk away, but I don't think there's any benefit to yourself in leading with it. If your initial attempt fails, then try something more drastic until you either get your desired result or quit.
I don't think you can expect people to be rational all, or even most of the time. Just because something is a fact does not mean it is necessary or beneficial to say it.
This is literally the point of the OP by the way, most people aren’t out to get you, and can be convinced of things if you communicate respectfully. No one wants to work with someone who goes out of their way to tear them down.
Never got this kind of behavior. I'd have told him that what he was doing was very inappropriate (without buy-in from the team), and if he doesn't back off I'll quit. He can ask around for other opinions so it's not just coming from me but it's a hard line.
Of course this mostly works cause as a developer you don't need to be afraid of finding another job. But that's good. Workers having leverage over bosses is good.