I think you are trying to say that if BTC becomes a global reserve currency means managed by governments then it might be manipulated or maybe go near to a centralized one. If that's what you are asking then let me clear a few points BTC is still being manipulated by many market makers and the second thought about centralization, then it can not be centralized anytime. Because it is running on different nodes placed at different places in the world.

If some government adopts it, then the price of BTC will shoot up like it did when El Salvador adopted it. and now I am hearing Argentina has also adopted BTC but has not made it a legal tender yet. Besides the decentralization factor, BTC is not capable of handling millions of transactions made in a single day, I have said many times before it can't even process the transactions of El Salvador's country if each individual made a single transaction in one day (as there is around 7 million population of it). So, it is not possible for the nations to adopt it as legal tender but they can make its trading and usage legal and it is in the benefit of the crypto market. But with rules and authorities, there come requirements which it had to follow.