20 years ago nobody would believe there will be something called digital currency (cryptocurrency) that will be threatening the very existence of fiat, same thing happened years back before the advent of fiat, most of the medium of exchange back then where slowly replaced with fiat.
Now with the technological advancement of this generation it is very likely something more advanced will be created, the world is evolving very fast, new inventions every now and then, so it is possible something new will replace crypto in the future.
No one can predicting how popular cryptocurrency as digital currency and now some of them most disappointed why not to be investor trough bitcoin and altcoin still have lower price. I think bitcoin will be effective few years later as legal digital currency payment transaction with billion member have been active as user or investor on bitcoin. All company will agree with bitcoin as legal currency payment depending what their costumer want, if still not try to adopt bitcoin I think several company will left by their costumer.