Earlier quoted context omitted.
The stock market won't collapse in the same way because stocks have earnings and dividends (which is what the value is largely based upon). Sure: securities can go up, or down, in price almost arbitrarily. But they largely can't go below zero (aka: bankruptcy law protects against that), and they can't really go below the expected profits of the company (because shareholders are entitled to those profits. Worst-comes-…
Invidiual stocks collapse all the time. Selling down large positions is a real problem that institutional investors face and they plan for a haircut on the list price when doing so.
Either expectations of future income, or present circumstances.