Simply speaking: bitcoin trades platforms are not a closed system.
There are people that buy to hodl it forever, in hope to become billionaires one day(it is like money thrown in the market but never out).
There are people that exchanges bitcoins for others cryptos, or other cryptos for bitcoins. Some never revert their moves, some brings back more when they are back, some brings more less(like I did when I tried to mess with altcoins).
There are lost coins, for any reason(lost private keys, owner convicted to life imprisonment, owner dead, etc...).
There is in fact wealth generated by bitcoin economy, like gambling sites, service sites, jobs, money from advertising, silk road sellings, etc. We all know that an economy where wealth is created is not a zero sum economy.
There are miners that sell their bitcoins, its like they are sucking money out of the market.
There are people that are paid in bitcoin for jobs, signature selling and stuff. Where is the "other side" of their gains?
If you are thinking about USD value of Bitcoin trading, than you can be right, if Bitcoin value will keep growing, which seems like the most likely outcome, than even those traders who have suffered loses would still be in profit
as long as they hodl enough Bitcoin to cover their USD loses. But since they have lost some of their coins, they have missed an opportunity to have more value in the future. So, Bitcoin trading is zero sum in terms of amounts of Bitcoins - for someone to get more Bitcoins via trading, other should lose some of their coins by selling at the wrong time. In general, this means that you should hodl some Bitcoin for long term instead of trading with all your BTC portfolio.