[throwaway210222]

This.

At some point the world economy is so much bigger than the USA's (even if no one stuffs up), the the US government simply CANNOT product enough dollars* for the demand.

Exactly the same reason why the Swiss franc can never be the global reserve currency.

* Although the current USG is certainly giving it a good go ;).

[hntrader]

The supply of USD should be mostly irrelevant. If supply is constrained then the FX rate just adjusts.

The parent's point (about US' share of world GDP) is quite different to the one you're making here.