2008 was not a reversal in the secular decline in interest rates and increase in leverage. Quite the opposite, it just ushered in the next leg. While you are correct that (a)+(b) can keep the party going under all conditions we have seen for the last 40 years and likely for the next 10, the trouble is that if you hit the (a) default button too much people eventually get tired of the inflation and force you to deleverage and reduce prices (in real terms). This is the actual bust.

Or maybe replacing our workforce with robots is actually deflationary enough to make it different this time. Who knows.