What do we do once there's no more money to print?
We can print tens of trillions and still not see inflation, if we control the costs of healthcare and education. One should not think about nation state budgets like a household budget. We’ve already spent $2 trillion on the first stimulus salvo in the US, and discussions for the next round are already taking place. The Federal Reserve has committed to “unlimited support”, so the decision has already been made; we’re just haggling over how it’s going to be spent at this point.
If so much money is printed (& spent) and (real economic) inflation is not seen, the logical implication is: a) The wealthy absorbed the domestic money in (real) asset price inflation (hence lower yields and higher wealth inequality). b) The money was "exported" out of the country to purchase foreign goods and services (leaving an interest rate burden that will eventually suck the "real" tax budget dry).
Looking at other goods and services, there’s only so much food, energy, clothing, and so on you can consume. Additional dollars beyond a certain point provide diminishing marginal utility. Bill Gates doesn’t order tractor trailers worth of food; he requires roughly the same amount of calories as you do.
So! Due to the transition to a services based economy, the US economy slowly but surely moving away from oil needed for growth (over the last 15 years), global demand for dollars, and structural demographic changes (an older population consumes less of everything except healthcare), we can print fiat without issue with some cost controls around inelastic needs.
At a high level, there are only a few productive economies in the world: USA, Germany, and to a lesser extent Japan and S. Korea.
The rest of the world consists of countries that are either small, subsistence farmers or welfare states.
So the USA can basically do whatever it wants monetarily. It also helps that it's the only global currency. You can go to the farthest ends of the earth, and people hoard US dollars there.
If third parties start using other currencies, printing money won't help.