No wealth was created or destroyed in the making of these millions, the millions simply changed hands. Think about that before purchasing a cryptocurrency. You're basically handing your money to an earlier "investor" in the hopes that a future "investor" will hand a greater amount to you. This is all there is to it.
Your characterization of all cryptocurrencies as a ponzi scheme purely based on the fact that people buy it because they believe others will buy it later at a higher price is weak because that is how all monetary assets like gold operates. We take it because we believe others will also take in the future.
I personally buy bitcoin not to sell for a higher price but rather, I believe strongly that a monetary system that's provably scarce, programmable, completely decentralized, permission-less, trustless, higly divisible, borderless and very easy to transfer has the potential to revolutionize how we store value.
This does not mean that everyone buys bitcoin because of this, some (perhaps majority) buy it because they believe it's going to go up but I see that as an initial incentive to get people into the industry.
At the end of the day, only time will tell if cryptocurrencies can actually live up to their promises of revolutionizing the financial system.