Diana can't just "decide to take her profits" at whatever price she'd like. If she wants $1m but Charlie isn't interested, she will have to lower her price until he is. This is how exchanges work, cryptocurrency or otherwise. If a stock falls to zero, do you think that Etrade dips into its own assets to provide an "off ramp" to its customers that weren't able to sell in time?

You're essentially saying that the exchange should artificially prop up the price of bitcoin with its own money during a panic.