Okay, hypothetically, let's say that bitcoin, or a similar cryptocurrency, is going to become the global currency that will replace all national currencies...

Could a global government, like the UN, build a server that would handle all the transactions, thereby making it impossible for private servers to turn a profit? It appears to me that bitcoin mining is becoming more an more centralized due to the increasing demands of staying competitive. Whereas a person could mine bitcoin on their laptop a few years ago, now they have to have very sophisticated purpose built servers and cheap electricity just to stay in the black.

So in the future, will bitcoin mining become so sophisticated that only a centralized entity will be able to handle it?

What I'm thinking is that if we get to that point, then the transaction fees will essentially be the equivalent of taxes. Has anyone ever proposed that scenario before, or am I way off base here?

ETA: Part of what got me thinking about this is that supercomputers are on the horizon. Could the UN simply come in with a highly sophisticated super computer and completely take over bitcoin mining?