>So in his opinion, it very much is NOT the US falling, but simply withdrawing because the economic order it used to bribe most of the world to be allied against the USSR no longer makes sense.

Correct.

Among its many "Trump is such a threat to America that I think America ought to be destroyed as a country to save it" flaws, I love how Fallows's article glosses over most NATO countries not meeting the 2% target commitment, as if that isn't the fundamental basis of Trump's complaint about the alliance. As you say, the United States basically bribed most of its allies during the Cold War to fight the Soviets, and among those bribes was the willingness to spend to make up what they couldn't/wouldn't spend on defense. Russia today is an economic pygmy that faces within a decade demographic collapse (another theme of Zeihan's). Its conventional forces only pose a threat to Western Europe because, again, most NATO countries—notably Germany—aren't anywhere close to the 2% target and won't be by the 2024 deadline. (And yes, the four states most exposed to Russia—the Baltics and Poland—do meet the standard.)

Over time I've come to wonder more and more if Jeane Kirkpatrick's initial reaction to the end of the Cold War—to get the heck out of NATO and return to traditional American isolationism—wasn't the way to go. The great challenge to America going forward looks to be China, and that's an area 1) explicitly omitted from NATO's purview and 2) the US has worked bilaterally, not multilaterally.