The US can't default on their promise to pay or there is a very high risk it will be lights out for them for world reserve which will be instant hyperinflation.

As world reserve they have had to spend into existence a metric craptonne more currency than their economy requires in order to lubricate world trade (google Triffins dilemma).

ie: For Turkey (say) to buy oil off Saudi, they need to sell something to the US in order to obtain USD first. When world reserve status is lost, suddenly Turkey doesn't need USD in order to acquire oil. The rest of the world is holding USD for trading purposes and those USD currently never go home in order to claim something from the US economy.

Now lets turn the lights off on USD as world reserve: Those now useless trade dollars will try and find their way home to the US and start chasing things milk, eggs, cars...anything in the real world that has value as the currency devalues. Meanwhile, the USGovt is having trouble acquiring its goods and services with its rapidly inflating USD and doubles down on printing under Executive Order 13603.

When this one blows, one doesn't want to be caught holding assets denominated in USD.