If they were so united and so willing to use one world currency, there would never have been any need of bitcoin itself, the problem is for one single country it's going to be impossible to cover up the 51% hash rate alone, and eventually, if multiple countries began to do this they would face clashes with each others and will never be able to form any consensus, moreover, it's not at all financially wise for any country today to make such a heft investment, because the returns aren't going to be that exciting for them considering that not a lot of bitcoin are left to be mined now. If countries would really want one world currency they would simple legalize bitcoin as a legal tender and they will be easily successful in their mission.
You've said it yourself. If governments legalize Bitcoin as legal tender, then there should be nothing stopping them from centralizing the cryptocurrency in its entirety. El Salvador "broke the ice" and it should only be a matter of time before other countries do the same. Imagine if every country abandons their native currency (Fiat) for Bitcoin. It'll be the end of decentralization as we speak. We cannot discard this from happening in the future if adoption for Bitcoin continues to rise at an accelerated rate. Hopefully the community will be able to keep Bitcoin decentralized, for the sake of people's financial freedom. Just my opinion
