I guess imposing a tax on every exchange transaction of bitcoin will be the
loophole for the government to put a tax on a decentralized currency.
They don't have to tax Bitcoin transactions specifically
All they need to do is to make legal an income earned through Bitcoin operations (that basically means legalizing Bitcoin as I said earlier). But if they legalize this income, in most jurisdictions that would effectively imply that you have to pay an income tax on capita gains obtained by buying and selling coins. And to make you pay this tax (i.e. catch you if you are going for tax evasion), they just need to control fiat in and outs that can be used to go into Bitcoin and exit from it
it's more easy than it seems, they just need to tax the exchange directly, then the exchange will apply a higher fee on your deposit or simply when you convert in fiat
this would be the best solution for them to avoid tax evasion, instead of chasing the bitcoin addresses, which is impossible