The point to the post you're replying to is that "defending its own shores" is not the only value derived from American dominance. A free and unimpeded military environment in the middle east is one where, say, Russia feels justified in blackmailing Saudi over OPEC disputes.
A withdrawal of forces from ROK and Japan means that they now have to turn to China for protection from the lunatics in the North.
A world without NATO means that Russia gets to annex Russian-speaking regions of Estonia whenever they want, just like they ALREADY DID to the Ukraine.
It's tempting to believe that there are no militarist nuts with guns in the modern industrial world, but it's not true. What is true is that they've been largely suppressed in the post-cold-war era by American military dominance.
Until now. I'll actually put this prediction down in writing: before the end of Trump's term, there will be at least one shooting war involving one of the players mentioned above.