as the economies of the world slow down,
the u.s. dollar will go up against their currencies.
Not really, it may even make the US Dollar worse. You have to remember what was saving the US Dollar from a total collapse was many foreign banks bought them. The US was the Gold standard of international currency. If any other country tried to do what we did, they’d end up like Zimbabwe.
So if the rest of the world’s economies start slowing down, then they’ll have less money to buy US Treasury bills. And with fewer customers, the US will need to either increase the interest paid on these T-bills, and raise taxes and cut spending.