> Plenty of currencies exist without being de facto global currencies and survive just fine. The question is what makes the USD likely to collapse as a currency altogether

Bingo! You got it! The USD is not going anywhere or collapsing. The global "reserve asset" named dollar however seems to be slowly on its way out.

Fair game right?

Problem is, there's a wave of trillions global dollars that will now seek to "go home". Inflation hasn't happened yet when fresh currency notes are "exported" (exchanged for real goods from the world) immediately. But those USD, whether in bonds or treasuries or bank deposits or hard cash, will seek to come back to their native soil when global savings or trades no longer desire them as they used to. Wait for it! The money has already been "printed", then "kept in custody" all over the globe for some amount of time (circulating at times, finding a good temporary residence for some other times), but still waiting to go back to their permanent home, to circulating with ever-increasing velocity..

The USD is the global reserve - therefore it is needed in vastly greater quantities to lubricate global trade than would otherwise be needed internally. To buy oil, France needs to export to the US in order to obtain USD to purchase off oil off Saudi Arabia. The US gets to print off (and spend) a lot of currency that would have otherwise caused inflation domestically but instead is mopped up by the need for global trade lubrication. Central Banks also absorb USD for reserves - for now. The USGovt is now addicted to its budget deficit that has been enabled by this reserve status but it gives the USA an exhorbitant privelige against other nations - this is starting to unwind - CB's no longer trust the debt of other sovereigns. There is a glut of USD in existence - vulnerable to a change in other nations willingness to hold them. The military is powered by oil and goods and services bought on USD. The USD enables the military, not the other way around.