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.