That's not really how it works either?
If I bought BTC at $50,000 and still own some, I've "lost" money but not actually since I haven't sold it. This is the reality for 100% of owned Bitcoins. There is $800B worth of USD in Bitcoin that has no "loss" side to it because at any given moment they are owned by people who have clearly not sold their share.
When the market cap goes down there is of course loss, but in Bitcoin's 13 year history it has gone from a $0 market cap to an $800B market cap, which means it is massively a net gain for people who have held Bitcoin.
Of course this changes if Bitcoin is revealed as the massive Ponzi scheme that so many HNers say it is / want it to be, but until that actually happens and BTC goes to 0, then as long as the market cap is above the cost of mining hardware it is a net gain for people who have held / continue to hold Bitcoin, not "negative sum". Negative sum would be:
(market cap - mining hardware) < 0
As pointed out by the other poster it is a bit more complicated than just market cap as market cap can be "gamed", but not really in the case of something with a healthily circulating supply, which is true of BTC at the moment.
