The interest on the national debt would increase to very large levels if they were higher. The government would then have to borrow more. They wouldn't find b

The interest on the national debt would increase to very large levels if they were higher. The government would then have to borrow more. They wouldn't find buyers for all the treasuries they would have to issue, so the fed would have to print money to buy the bonds to make sure there wasn't a failed auction (a.k.a Permanent Open Market Operations). This printing would work, for a little while, but all the interest paid on the national debt would go higher than the magic level of about 8% of GDP in which case it's becomes unable to profitably reinvest itself and instead beings to flood into foreign exchange and hard assets causing severe inflation. If the fed did not print then there would be a failed debt auction and a debt default. That, or there would have to be an unacceptable massive federal budget cut or a likely counterproductive tax increase to cover the interest.