Firstly, while total debt might have quadrupled, in that time the size of the economy has also grown, and that means the debt has become easier to finance. Debt sits at about 130% of GDP, and was about 68% of GDP back in 2008, meaning when measured against the size of the economy it has not yet doubled. Also, by comparison debt to GDP hit 116% in 1946 and managed to return to 31% of GDP in 1974, showing that given the political will, the USA has no trouble paying down its debts.

If the majority of debt was going towards infra, building new factories or improving old ones, reducing cost of housing/health/edu of workers, its all cool. But for a long time now, a mega machine exists that diverts it towards real estate asset inflation, stock market asset inflation, monopoly power consolidation, interest and rent collection, financial sector fees, buying national assets of poorer countries, startup ponzi schemes etc etc. The parasite is fantastic at generating lot of activity. The question for the US has always been how will the parasite be confronted and when.