US 2025 toy model: income growth 3.9%, effective rate 3.5%, debt/revenue 580%, primary deficit 12% of revenue. At these starting conditions, even the ‘favorable’ r < g scenario leads to 648% debt/revenue by 2035. If private demand falls and rates rise, the path to crisis is faster — the model shows a self-reinforcing interest rate spiral once private buyers begin demanding higher risk premia.