US debt has no theoretical limit. Instead debt value is limited by the amount of inflation incurred.
The way the US rents the finances from the FED is confusing and I'm not sure what it means for where the rent goes. Ideally, it just reduces the amount of USD in circulation.
The US is not in the situation that it's rent outweighs it's income, and is therefore not killed. Even if it does it mostly means the government and people will have to adopt another currency that's not hyperinflating. Then wait for the USD to devaluate the entire debt to a payable level. (Note: something Bitcoiny would be a pretty good fit for many reasons)