The fact that we didn't jail/banish him after leading an insurrection against the US Capitol was the "too far gone" moment.

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.