It's not that they don't consume, it's that that don't consume to the same degree of past demographics (they simply can't). That consumption would have worked its way through many hands (businesses & employees) causing a multiplier effect vs going to pay back a loan. So larger scales of student debt, aren't just larger numbers on the balance sheets, it pulls money out of the active economy.

If they can't afford to buy a property, a whole cascade of secondary 'new place' purchases don't happen.. it's a symptom of not enough circulating money at the center of society to sustain the economy.

The crash part comes when investors look at falling activity in China, and falling commodity prices as strong indicators that global demand is falling.