So much pussy footing in here.
Yes, your wins are at someone else's expense. Easiest case, if you buy the lowest point of a bear market, someone sold those coins to you and is missing out on the profits from the subsequent rise (Losing). That means, whether they were in profit or not when they sold, they still missed out on further wins. Now, if you sell the absolute top before the bear comes back, obviously he who bought your coins loses.
All this talk about "if you wait long enough..." No! Not how this works at all. I don't care who you sold to, even at the top, even if they wait for profit on the next rise, PLENTY have lost to pay for your win. Trading is zero sum, period. Just the way it is. It do dat.
Lets do a little thought experiment.
Player A, player B and player C all buy in at $1 with $100 of capital. Price rises to $10 where player A sells to new player D for $1000. Price keeps rising to $15. Now the score is A=$1000, B=$1500, C=$1500 and D=$1500. Player A's missed profits of $500 went to player D and player D's initial investment of $1000 is where the profits for B and C come from. $1500 in profit total since $10 price.
Should player A buy back in at $15, he now has 50% less coins and 33% less value had he not sold early. So lets say players B and C each sold 30 and 20 coins respectively to player A and then the price tanks to $7.50. The new score is Player A=$375 or 50 coins, B=$525 or 70 coins (AND $450 cash), C=$600 or 80 coins (AND $300 cash) and D=$750 or 100 coins. losses for this round are A=-$625, B=-$525, C=-$600, D=-$750, and new total valuation is A=$375, B=$975, C=$900, D=$750.
Now think of the situation where Bitcoin is found to have a fatal flaw. The first few people get out with something, the rest are left holding a bag of useless 1's and 0's as the fiat evaporates from the market. <-This is also why "Market cap" is the absolute worst thing to derive the value of Bitcoin. There is no way everyone would ever get out for anything above $1.00. All the bid depths combined would only allow one or two large holders to exit fully.