I'm going about 90% of the way with you on this whole post. Very well reasoned indeed.
But there is a but, and it's a big but (as opposed to a big butt).
The but is that we used to be able to do those long term plans within certain parameters, understanding that our counterparts in the United States would act within certain bounds. There was plenty of variability at play, but Trump tossed all of those bounds out the window. Fair enough, he's entitled to (until SCOTUS tells him otherwise because congress currently in lap dog mode).
And you are right Trump only has 2.5 years left. But there is no guarantee that the next President will be less of an asshole. Trump has proven that his formula works in American electoral politics and others might actually believe in Trumpism or they just might want to emulate it to get elected.
Unfortunately the world's future planning now has to include the possibility that the next one might just be another Trump. We might be pleasantly surprised, we might get another Reagan or even a Bush, but if we plan on another co-operative partner and we get another Trump then we will have made a critical error. Our forward planning has to include the very real risk that these trends might run much longer than the current administration. Doesn't it? Canada's and other countries turning away from the United States, if they can, is not just to deal with Trump, it provides insurance against a future Trump.