Is there even reason to expect exchanges to play along if the value starts to really go south? It's pretty obvious that exchanges would be practically out of business after a total collapse of bitcoin valuation, so a collapse would be an existential threat to them and existential threats are known to inspire desperate measures. What regulation would keep market makers from experiencing "unexpected technical difficulties" in a panic (e.g. until their own holdings are liquidated, and those of their friends)? There are many ways to add arbitrary friction and hidden prioritization to sell offers if you are not under the full scrutiny of all that regulation bitcoin aficionados want so dearly removed.

If you were forced to find buyers for your btc on your own in a panic, without the help of market makers, then collapsing nominal market rates would not be the main problem of "btc millionaires", getting that collapsed rate, or even just a significant fraction thereof, would.