I think the government is going to tax BTC as a commodity, which will insure it can't be a unit-of-account because legal tender is not taxed on VAT and capital gains. And then merge Bitcoin into some digital fiat plan...
I think you are out of your mind if you believe any such government-bankster hijacking of Bitcoin will be even remotely successful or tolerated by the cryptocurrency community.
World governments will have no more success taxing Bitcoin than the US government has had stopping the spread of piracy or winning the insane war on drugs.
And proper decentralized exchanges are coming sooner than you think.