Cryptocurrency and fiat money don't has intrinsic value, so the combination of cryptocurrency and fiat money will not give significant advantage. In term of comparison, cryptocurrency is better than fiat money because cryptocurrency is faster, safer and tye cost for maintain it is lower than fiat money. Cryptocurrency should be combined with something which has intrinsic value like gold or silver. We should reduce the usage of fiat then start to use bitcoin as the main of medium of exchange which and also pretend it as representative of gold.
Gold and silver are precious metals but they are not used in real industry, so their intrinsic value can be speculated and manipulated on markets. As I can see, the prices of gold and silver are slightly falling month by month, therefore rich people prefer to invest in more profitable assets.
In my opinion, it is almost impossible to replace national fiat currencies with crypto currencies, but these types of currencies can be combined and diversified. Crypto currencies might be useful for individuals from all over the world to make cheap and secure transactions from one country to another country.
It should be noted that nowadays many people use the Lightning Network. In fact, this technology is based on the fulfillment of the mutual obligations of two owners of BTC coins.