It’s not worthless. The United States, which happens to be the largest economy in the world, requires taxes to be paid in dollars, regardless of how you’ve settled your transaction. If you and I trade a car for 40,000 chocolate bars, the IRS demands we figure out the dollar cost and pay them their due.
If I owned all the dollars, the US would simply decide to start demanding taxes in something else because it could not use those dollars for anything. If it refused to switch to another currency, it would not be able to pay the people who would force me to pay taxes, so there would be no reason for me to pay taxes.
The US Debt is denominated in dollars. The USA has promised to pay its entire $24+ Trillion debt, in dollars.
But that would not make any sense if one person held all the dollars. The creditors would have no use for dollars, and the US would need to switch to another currency in order to be able to pay its employees. They could print however many dollars they wanted, and could pay back the debt that way, but that would not solve the problem of the dollar no longer being useful as a currency. The government's employees would have no reason to accept the dollar because nobody else would accept it. The dollar is primarily valuable because it's used as a currency. If it stopped being used as a currency, its value would be zero. This is no different from Bitcoin.
The United States can print more dollars. Hell, they can declare all your dollars not real and throw you in jail. Every dollar is owned by the US Government. You’re just borrowing them.