The valid currency USD as you have termed is in its last stage as global reserve currency.Its an inflation currency.It is being printed unlimited and sent for circulation.USD just gained its importance after OPEC countries decided to sell crude oil only for USD.Otherwise,this so called valid currency would have been nothing.
Bitcoin is not a ponzi scheme.It has some unique features for which it gets the trust of people world wide.Its true that it has given huge profits to its early adopters.But it is not now in its end phase.Just it has crossed its initial eight years and yet,a long journey is to go.Today's investors will get their returns tomorrow.
If you believe this, you don't understand cryptocurrencies. By definition, they're set out to die eventually. The amount of coins that can be produced is limited, and if you consider that many things result in frozen coins (e.g. deleting your wallet), it's only logical that after all coins have been made, the number can only go down, which will eventually reach critical levels and make the whole system impractical. That's the equivalent of the ultimate fate of the universe for a crypto, and with IRL money that doesn't happen because you can always print more. That doesn't matter much, because in addition to being inherently finite, it's also a speculative bubble that will burst; proof is how unstable it is when reacting to supply/demand (e.g. that one time weeks ago when it dropped $2k because some people shilled another coin).