They’ll find a way to do this. Default means you lose whatever power you have in the world, and risk a world war.
No kidding- a default is very serious. A partial default wouldn't be as drastic as you say though. I wonder where the credit rating agencies are right now... we went very quickly to $25T in US sovereign debt with decreased revenues (both because of COVID) and they're being just as quiet as they were back in 2008 about the banks.
But I don't understand the quote in your post, we already sell long term debt instruments. Bonds come due and instead of paying them off out of revenue we raise the funds by issuing new bonds, so future servicing costs depend on current interest rates, it's not possible to "lock in today's rates" to answer that issue.