Everybody _does_ know, however, that the US government cannot keep borrowing money forever.

I am not one of the everybody who knows that, as I know the opposite to be true. Real debt can definitely rise forever without any problem at all, and generally should. But a country's debt to GDP ratio presumably can't rise forever. The trick is to keep an eye on the gap between the two rates of growth.

Even a quite small move in the rate gap turns a certain foretold disaster into a "oh, look, no prob". Canada was "doomed" once upon a time, around the early 1990s. The national sales tax was rejigged and the problem went away.
I'm not sure whether this link will work. https://cupe.ca/sites/default/files/gdp_en_0.jpg
(CPP and QPP are the net national and Quebec pension fund assets, respectively, and should probably be excluded for this conversation because they are there to attempt to cover known future liabilities, so use the top line). And Canada had no advantages from being the main reserve currency.

Jim