First, we are in the era of sovereign countries. Currency is a symbol of sovereignty. Monetary policy needs to regulate the economy, while private cryptocurrencies are decentralized and decentralized, and the government does not have the right to issue and control the currency. It is very difficult for a country to give up such a sovereignty.
This is not a valid argument because bitcoin is not designed to replace fiat currencies but instead it is designed to offer an alternative option that could be used alongside the centralized fiat currencies that each country has for its own. As a borderless censorship resistant payment system.
Second, the conservation of the total amount of Bitcoin makes it face a dilemma.
It is not a dilemma, it is an advantage compared to the inflationary fiat currencies that keep being printed to solve the economic issues governments face at the cost of regular people.
Many studies have shown that Bitcoin is now concentrated in the hands of a few people,
You mean various FUDs have said as much otherwise there is no truth to that. In fact as time passes, bitcoin becomes more decentralized. It was even proven recently that the biggest and longest existing FUD was a clear lie. The FUD telling people that "China controls bitcoin by having 90% of the hashrate". The moment China banned mining, the hashrate change was negligible!
Therefore, it is difficult for private cryptocurrencies to become the legal tender of most countries, let alone international currencies.
Bitcoin is not supposed to be come legal tender of any country, but it is and will always be an international currency.