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.