If wealth is a function of ownership why would you care if your $500m went down to $100m if the purchasing power you cratered is well below what you lost in 'value' while your ownership increased?
If the global economy shrinks by 80% (using your numbers) then their relative wealth can remain the same. If the US economy shrinks by 80%, then their global wealth has substantially declined while the wealthy in other countries do not suffer as much.
That would only work if they have large cash reserves, we have data that shows most billionaires/millionaires don't their wealth is tied up in assets.
Because of that if the market does crater their businesses/assets may in fact get wiped out.
The rich don't want volatility or crashes, it rarely benefits them.