Wealth went down, but you've only given one example. Asset prices can change in response to asset values changing(like your example), estimates of asset values

Wealth went down, but you've only given one example. Asset prices can change in response to asset values changing(like your example), estimates of asset values changing, liquidity problems, and risk preference changes. And it can feed on itself. In some of those cases real wealth is lost, in others, it's just people throwing around different bids and offers.