It's not what Trump will do, it's what domino's he knocks over and what his messages set in motion elsewhere - because of what he's said he won't do and what agreements he's said he will renege on..Ironically, the very thing he claimed made other people stupid when dealing with foreign affairs.
NATO is one of the most important partnerships that's ensured peace in North America and Europe for many decades. He's undermining it to the extent that it could all unravel and that would be a very ugly future for everyone.
He's also likely to drive countries like Iraq more towards an emboldened and reverting Russia rather than closer to the US (yes, as potential allies - shouldn't that b the goal?).
This hits the nail on the head. Trump has promised to do all kinds of crazy shit and in particular cast doubt about what he will do. It is that uncertainty which is dangerous: if everyone knows NATO will go nuclear apeshit at the first given chance over even the most insignificant piece of land (as were the case during the cold war under eg Reagan) there is no reason to test that. Now that trump is like: hum dum, how much are they paying? That is an open invitation to see what will actually happen for eg putin. On top of that, by all accounts trump cant hold his shit together when a comedian sayes his father was a monkey, how are he going to handle real opposition from a smart diplomatis team who tries to trigger him?