The first is that other Western countries could pick up the slack instead of having it fall disproportionately on the US. It only got the way it is because the others were mostly bombed out after WWII and that's no longer the case.

The second is that if the US spent less and did less military interventionism, adversarial countries might not feel the need to spend as much either, and then the balance of power stays the same while everyone spends less money on bombs and warships.

The US is the only one who can initiate this because they're the only ones who can reduce military spending without being threatened even if others don't.