Interests rates do not need to increase very much at all at this point, so that 100% of tax revenues would be needed just to cover the interest owed by governments. At this point (actually before) the currency is effectively dead and hyperinflation is inevitable, unless:
1) there is a draconian cut to government spending to match tax revenue, or 2) the current system is abandoned to one that is backed by hard assets (which would include a severe cut in spending but could also include a taper)
1. is politically infeasible, so we will default to 2. after a significant amount of economic pain. The US Dollar and most western fiat currencies are dead and are already long in the tooth by historic standards.