First, we can consult the Trump Rules and focus on projection, numbers, and, yes, entropy.
- Trump has a hard time valuing reciprocity, NATO' essence via the promise of an attack upon one is an attack upon all, because he projects onto everyone else how we views things. That other countries must be ripping the US off and trying to get by via doing less precisely because that is his life-long modus operandi. He just does not get the security guarantee because he can't see anyone else keeping it since he never keeps his word. So, NATO can't mean as much to Trump as it means to damn near every major politician in the US and Europe for 7- years (De Gaulle pulled out of the operational command, but he did not pull out of NATO).
- Trump can't get any numbers right, so, of course, he does not understand that the 2% guideline--that members of NATO should contribute at least 2% of their GDP to defense--is not some kind of payment to the US for the costs of defending Europe. Nor can he understand that not everything the US spends is on European defense. Nor that 2% is a dubious measure.
- Trump does not value order. He likes chaos in his own team, fostering rivalry among his advisers. He does not value European order either, seeing the allies as competitors. It may not always be deliberate, but it is part of a basic tendency--to dismiss institutions and norms and processses. Maybe sometimes we overvalue such stuff, but avoiding major war in Europe and fostering prosperity has seemed like a pretty good deal for 70 plus years.
Third, well, we could consider the Putin factor. Not so much that Putin is causing Trump to behave this way, but that Trump was an attractive candidate to the Russians both because he was a force for disorder and he was hostile to NATO. One of the basics of Russian foreign policy under Putin has been the desire to break NATO. No need to invade the Baltics to test NATO if the arsonist in the White House is trying to burn it down.
Could the US survive without NATO? Sure, its nuclear weapons mean that only terrorists will directly attack the US. But will the US thrive without it? No. Tensions, crises, and friction in Europe will come home to roost. Consider how often the US has been involved in the Mideast, which is actually at best the third most important region in the world for the US in terms of trade, investment, and all the rest. Europe remains important even as China replaces it as the second heavyweight in International Relations. NATO has been a good idea for more than 70 years because prevention is, indeed, cheaper than the cure. We have avoided a third World War, and the stability that NATO has brought to Europe is a significant part of that. IR scholars disagree a lot, but they definitely agree on NATO
As we enter an age of multipolarity where things become less certain and there is more room for misperception that can lead to war, it makes even less sense now to give up on a key source of certainty and stability. But then again, everything Trump touches turns to shit. Will the Republican Party let Trump sell off one of the most important assets of US national security? Probably.