Are you talking paper gains or actual cashed out real money gains?
Bitcoin is virtual currency based on massive electricity consumption. Due to the ever increasing difficulty of mining, it is effectively a Ponzi Scheme.
Difficulty is not in any way ever increasing. Difficulty is set based upon the overall hashing power of the network. If it becomes more expensive to mine, miners drop out and difficulty decreases.
The worry is that when the crash comes, no one will be willing to buy. Or even if you find buyers, what if everyone starts withdrawing their money from the exchange, and the exchange doesn't actually have that liquidity...