Hey there, great question!

Well for a Ponzi scheme there should be somebody promising returns for an investment, take your money, then distribute that to everyone who is waiting for their returns.

When you buy bitcoin, the exchange usually just gives you bitcoin but doesn't promise you anything, nor is your fiat money then distributed to other bitcoin holders.

Holders with skin in the game may claim it will go up, but have no control over that beyond not selling theirs. And they aren't paying anyone either. 

If you would sell for fiat again at a gain in fiat terms, you just made a profit comparable to any other investments in stocks, gold, whatever.

BONUS: If you're in bitcoin long enough you'll start to see that bitcoin is, in fact, the actual money you needed all along to the point that the price stops being very important... think about that one.