You are right on saying that. However, I think that although crypto might not be a good alternative to fiat and such payment methods, the decentralization is very much needed in the modern world as institutions and businesses are making the wealthy wealthier and poor poorer by their manipulative techniques. Every pros come with cons, it's just how we manage to milk out from it.
Decentralization is needed and it is acquired by the people. I am against your idea of it cannot be act as an alternative of fiat, I am of the view that not in future, but in present time it is acting as a best alternative of fiat currencies. People have adopted it as an option at all places where it is acceptable as a payment method. People are also investing huge amounts in trading with digital currencies. These are the results of its usage and acceptance by the people.