If the debt got to a level where it could not conceviably be paid down from taxes, that could cause the dollar to fall. Yet US taxes are remarkably low compared to other OECD countries. The tax take could even double from where it is now without completely squashing the US economy, although that would be severe. Also, the Fed can put interest rates artificially low if they felt they had to do this - there would be less buyers though. It's a scary balance sheet no doubt, and the long-term result doesn't look good to me.