I think you're being a bit overdramatic. American military power is important to western security, but it's not everything, and I don't see a President Trump suddenly turning around and allying with China to bring the world to its knees. A return to American isolationism wouldn't be a good thing but it also wouldn't be the end of the world.
Military power? No
Economic power? It's the fucking lynchpin of the western economy, that's why shit like the GFC could happen. The US needs to stay stable, above all things, so that the rest of us can sort shit out.