The math on the trajectory is straightforward — the harder question is who provides the bid at 70-80 trillion. At current rates, interest expense alone crosses 2 trillion annually well before 2030. That's larger than defense spending and approaching Social Security. Individual investors rotating out of risk assets to buy Treasuries is one source of demand, but it's a finite pool and it requires sustained risk-off sentiment. The three structural buyers are the Fed (monetization), foreign central banks (geopolitical alignment), and domestic institutions via regulatory nudges (financial repression). Each path prices differently. Monetization means dollar debasement. Foreign demand means geopolitical concessions. Financial repression means captive capital and lower real returns for pension funds and savers. The market hasn't decided which path yet — but the bond market's term premium repricing suggests it's starting to assign probabilities.