Then I thought about it a bit more, and realised (and the last four years have shown) that if America withdraw, the Russians and the Chinese will move right in, and while I'm not a major fan of American imperialism, I'm even less of a fan of the Russian or Chinese flavours.
And the US built the UN and all the other multilateral organisations post WW2, so yeah they work for the US, but they're also pretty good for everyone else.
I owe that to hundreds of years of human progression and development, not to the US.
Not to mention the Cold War, which only stayed cold because of the nukes holding both sides back.
[1] https://en.wikipedia.org/wiki/List_of_wars_involving_the_Uni...
Pax Americana usually means the same thing, though the borders of the empire are a little less clear.
If you review history, large wars broke out between the major powers vying for dominance on a regular basis in the 19th and early 20th century, culminating in the First and Second World Wars. For all the negatives of what the US has done with its military since then, they're not a hundredth as bad as what went on before by any objective measure.
The Pax Americana has been good for stability and with stability comes prosperity. If the United States truly gave up its hegemony and turned isolationist, you'd see a resurgence of rearmament and jockeying for power among nations, inevitably followed by a new era of war after war.