The US Gov will never have an issue paying their debt because they are sovereign over the currency they use (USD). Furthermore, they have an extra advantage because the USD is the most popular reserve currency and because of this demand around the world, it's easier to maintain its value or relative strength to other currencies. That being said, long term it's not looking good for the USD. If you check the DXY index, the USD has been trending lower against a basket of currencies, and the FED had to buy a lot of treasury bills in 2020, because of lower demand from other entities. Another bad thing for the USD is the long term liabilities of the US Gov, which will only be able to be satisfied by printing dollars. (https://www.youtube.com/watch?v=YpgfIfD_cRs)