Future freedom of action. Being in debt can constrain your future options. If nothing else, it can reduce peoples' willingness to lend to you later when you need them to.

People sitting around doing nothing constrains your actions.

In a country which prints dollars, and borrows dollars, future debt cannot constrain action.

The fear here is whether people will stop lending you dollars, but as we can see in the US we are nowhere close to that situation. (Europe/Germany, which is pathologically committed to not having debts has it even worse/better...people are paying them to take their money).