That's pretty flawed statement. Cryptos are alredy proven to work as a payment system, because you can pay with them.
Oh really ? I don't even know of a single shop in my country that accepts them. Only a hand few of online retailers accept them and the list is shrinking quickly. Big parties like Steam have stopped accepting them because of all the problems mentioned.
Where did you get the idea "it's all or nothing"? Explain why Bitcoin, or any other crypto, is only worth something when it has the capacity to service every person in the world? Why would it have no value if stays forever in a niche?
Do you really believe that some shops will continue to accept them, if pretty much none of their clients is using it? Why go through all that trouble?