I think very likely the future doesn't have one reserve currency, that the USD still plays something similar to its current role but similar transactions also happen in more currencies. so we lose sole reserve currency status, but not reserve currency status. The truth is we are already past this point. A lot of oil is already being traded outside of the dollar system already which wasn't true even 10 years ago.
There are 100s of hours of people including Gromen discussing this topic in depth on youtube if you look around.
It's more accurate to say: you don't default on your debt when your debt is denominated in your own currency. It doesn't require any specific reserve currency status to play that game.
There is the Euro, but it's not like there aren't several EU countries with severe financial issues. COVID hit the Eurozone pretty hard, and with high inflation, the war in Ukraine and the prospect of very high food and energy prices the next six months I wouldn't be too sure. Interest rates are finally going up aswell. EURIBOR just went above 0% for the first time in many years and there's a lot of people in overvalued homes currently fearing the bubble will burst.
1) Things are about to get incredibly nasty over the next 3-6 months.
2) Things only stay bad until the Fed capitulates and decides to start rescuing things. In this case we will be choosing inflation over a recession. And I'm saying inflation as a long term trend, at least 5 years.
But basically the game is up. This all ends up in one of two ways
1) The fed defaults on the debt and tells the Boomers they were just joking about Social Security and Medicaid existing as interest expenses drastically increase while tax receipts drastically decrease from a baseline of 127% debt to GDP (this will NEVER happen. You don't default on your debt as the world's reserve currency. Why would you? This is the one thing the MMT crowd gets right)
2) We give up and take our inflation medicine to cure our sovereign debt issues.
It's going to be option 2 11 out of 10 times. History is pretty clear on this point.
To me, no one lays this case out better than Luke Gromen:
https://youtu.be/VCAe4lVEpgs?t=37