I don't think you know what a ponzi scheme is. A ponzi scheme is a situation where a business pays it's previous investors with the money from it's new investors... In a fairly launched crypto currency there is no business or other central entity distributing the coins so there is nothing to invest in and no one to pay you back. The only think you can "invest" in is the network itself and the only thing you can "invest" is the work of your computer to secure the network for which you will be rewarded with some coins. Every other good store of value in the world works the same. In order to obtain gold or silver you have to pointlessly dig in the ground searching for some useless metal whose only worth comes from it's scarcity and being difficult to obtain. In that sense if crypto currencies are a ponzi scheme then so are silver, gold, diamonds, €/$/£ (paper or digital), stocks and everything else whose value doesn't come from it's intrinsic qualities.

If you don't want to dig in the ground/use your computers work you can pay someone else to do it or just buy the metal/coin from them if they already acquired it. But if it's so useless why would anyone spend their time, effort or intrinsically valuable things (like food, fuel, tools etc.) to acquire it? Because while it's basic qualities don't make it a good source of energy, food, heat, light, shelter, security, comfort, entertainment... non of the things we as humans value, they do make it an ideal candidate for a store of value a unit of account and a medium of exchange. That's why people valued this metals for millennia and continue to do so. They don't have value on their own, but in the context of the societal system we live in their intrinsic qualities make them invaluable. The value of gold, cash etc. came from it's place in that system, crypto currencies are in many aspects an improvement on those intrinsic qualities that make gold and cash valuable so it's only natural that they will replace the aforementioned in many areas of life.