What is wrong with this line of thinking:
Virtually all cryptocurrencies that are intended to be used as a replacement for fiat currency will become somewhat worthless.
All cryptocurrencies and their underlying technology are simply stage 1.
Stage 2 is where, once the technology has advanced enough from other peoples efforts, the Government creates its own either universal or country-based digital currency.
Afterwards, no one uses any other form of cryptocurrency (as a form of cash), except some of the techie-types or for purchases such on the dark web, because it is not the mainstream digital currency.
Some governments are already hinting towards the creation of their own cryptocurrencies, and several groups already gave negative remarks about it. There will always be a distinct difference between decentralized cryptocurrencies and cryptocurrencies ran by a state: trust. Decentralized currencies are trust-less, and only has a certain number of coins that can be minted. Every transaction records is publicly displayed on the blockchain, so anyone can view every transaction if they so do like. With this, you can
rely on the fact that the value of your coins isn't destroyed in any way by artificial inflation or insertion of new money supply past supply cap.
For government crypto, you cannot be sure that there aren't any coins that are magically inserted since transactions would be private, and only the state can know and view how much money there really is in total circulation, just like what they are doing with fiat. Government crypto is easily controllable, susceptible to hyperinflation and can be manipulated depending on what the government has in mind.
The government could also enact laws to prevent business from accepting any other form of digital currency, or make deals with payment processors to stop them from allowing other cryptocurrencies to be accepted.
They already are.
Bitcoin may hold some value among collectors, as it was a first mover. Also, cryptocurrencies that are tied to a certain company or utility can hold value, such as how stocks function today.
But any fiat replacement coin besides the one adopted/created by government, ultimately will be worthless absent within some fringe communities.
At present, no one really knows what would happen to decentralized cryptocurrencies. Ultimately, every currency would be worthless once this planet is shattered into pieces by wars for money and power. In the short-term future though, the possibilities for bitcoin to flourish and remain are bright. With the growing pains of the world towards their own governments and currencies, it's not impossible for people to turn towards decentralized currencies and gold for safe-keeping of assets and for payment purposes.