Overall it's not bad that national debt is there. Because usually a nation gets something for it. And national debt is paid back, all the time. Nations don't default on their debt that often, if they do it's really, really bad.
So overall, debt isn't paid back. The particular debt is paid back very much indeed. And whatever was paid for with that debt is ideally here to stay. National debt isn't anywhere close to private debt or a mortgage. Looking at it that way is just wrong.
By no account do I understand monetary theory or how national debt works in detail. I know enough to understand that it is a very complex topic, one that isn't easily understood. So I am cautious when it comes to easy explanations.
That the national debt will never be "paid off" is hardly inaccurate. It's barely even controversial. It wouldn't be a good idea either.
Hell, I remember the news articles from the 90s during a brief period of budget surplus that fretted about what pension funds and insurers would do if they couldnt buy t bills any more. The mask briefly slipped as the reality of the national-debt-is-basically-just-a-savings-account before we returned to our regularly scheduled implied bankruptcy doom-mongering and budget ceiling theatrics.