I think U.S. allies had long since internalized that they would occasionally have to eat shit from the U.S. The bargain was a place as a vassal state instead of a target, and if those are your choices being a vassal state has a lot of appeal. The occasional overt screwjob is much less damaging than a constant destabilization effort.

The deal will continue to get worse under any U.S. president, but what they seem to be getting at here is the possibility of it getting torn up altogether, opening the door for more direct U.S. hostility. As long as they support NATO they aren't likely to be the target of a coup like the 2014 one in Ukraine, but what if NATO is gone?

Trump isn't going to be allowed to unilaterally withdraw from NATO on a whim, but he could do a lot of damage to it, and he can rile up the reactionary hogs against it, which would at least lay the groundwork for a still more impactful change.

Europe has been a vassal since 1945. How do you vassalise your vassal?

EU suggested forming an EU army as early as the 1950s! USA was against the EU forming its own integrated military. So was, not a member yet, Britain, historically always scared of a too united continental neighbour. Instead, the Anglo-Saxons basically forced the young EU-predecessors to rely on NATO instead of forming their own big military defence force. How the tables turn.