That's starting to sound a bit no-true-scotsman. If we need an existential threat to the US, that's not going to happen - realistically China conquering Taiwan or even building an empire around the Pacific would still not be felt as such a threat.
The US is already close to losing world hegemony status and it kinda needs it in order to print money / export inflation. A multipolar world is one where the US is greatly diminished and this will happen with or without losing a war.
Like what though? The failure in Iran has had pretty substantial consequences that are being felt. If that's not good enough, what is? You were talking like you thought there was a realistic path to a better military, but consequences for the US aren't going to come much bigger than this.