I think that the foundation of fiat governed by the government is not that easy to replace. If by chance, the economy might be at stake. Imagine how would a country fund itself if all of the people use decentralized currency. Taxes and fees may be a bit off but still we do know that some have a cause. We just see it as a bad thing most of the time.
I know both fiat and crypto are very in contradicting to each other. But coexisting makes it beneficial.
I would say crypto is an alternative but not a replacement.