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.
There were digital currencies even 20 years ago. I mean they weren't as big as bitcoin but that is mainly because of blockchain, bitcoin became the digital currency everyone accepted because of the backing technology it had with blockchain. So, everyone who worked on such projects knew that one day it would happen, didn't know when and didn't know how it would happen but they knew that it would one day be perfected and happen.
The first ever happened in 90's, we are in 2022, that is over 30 years of trial and error until this day, but if we look at 2008-2009 period as the start, that was a decade of studies done on how to perfect it as well.
Correct, those that know about the topic knew that a decentralized currency was possible, however it was not until several technologies were developed and Satoshi put them together in a masterful way in which something like bitcoin finally materialized, so while without a doubt bitcoin is the latest iteration when it comes to money, it would be a mistake to think it is the last step, bitcoin without a doubt will be improved upon but at some point in the future I would no be surprised if something way better appears, however I do not think this is something that will happen soon.