The reason there is no unringing this bell is not just that we have a capricious, vainglorious president, it's that all of checks and balances that are supposed to restrain the executive have proven worthless so far. Republicans in Congress have completely declared their impotence, having fully relinquished their duties that the Constitution specifically delegates to the legislative branch, like tariff power, war powers, etc.
Just as we've seen in the car industry we'll wind up less innovative, less productive, and less economical.
The fact that we didn’t immediately remove him after the Greenland letter just demonstrated to our allies that that same fraction of people also don’t value our relationship with Europe.
The CCP looks thoughtful and stable by comparison. It’s too bad that the result of this will be to force more people to align with them, because they’re just as self-serving as Trump, they just have the good sense and temperament to hide their true intentions.
Anything after that, is just Yet Another millimeter along the same trajectory:
Well, if we let him get away with J6, then I don't see how this is too much worse.
To tie this to your point, I honestly don't see why the Greenland letter should suddenly be the straw that shatters this particular dromedary's spine.