If governments legalize Bitcoin as legal tender, then there should be nothing stopping them from centralizing the cryptocurrency in its entirety. It'll be the end of decentralization as we speak.
I don't understand what made you jump to that conclusion. If governments all over the world suddenly made bitcoin legal tender, that would force almost all population, that is, banks, businesses, individuals, merchants, etc., to switch to that new legal tender. Not only would it result in a higher price and greater awareness of bitcoin, but also it would result in more people willing to mine bitcoin or run their own bitcoin full node, which would make the bitcoin network even more distributed and decentralized. It is pointless for governments, especially big ones, to promote bitcoin by making it a legal medium of exchange because once people learn about how to be their own banks, they will no longer need to rely on centralized government-controlled entities. Why would they shoot themselves in the foot?
Exactly my sentiment as stated above. However I would also add the fact that if a government someday, somehow manages to centralise Bitcoin, then there will be other crypto's to take its place. Especially privacy coins. So trying to decentralise Bitcoin is not something that the government can do, I believe. And attempting it in the first place will only lead to a waste of money and time.