In my country, I have to fill a form every year for the tax department, including my year end bank statement and the salary bill.
Together with some major bills for insurance and healthcare, that's all they get.
So I can hide cash under the bed, or have an overseas account, or have some Gold bars, etc. The goverment has to thrust me quite a bit.
They can check some transfers, but if you like to hide it, you can do it.
So the actual way of getting the real numbers is quite bad.
In Bitcoin
If the Tax department makes it a rule by law, that you have to report every BTC address you own, or every wallet you own ...
All problems solved, because then the Department has also the addresses of the petrol station and the deli you spend money.
In no time the system knows which tax counts here and can claim the respective amount from both sides.
Sure it is still possible to hide Bitcoins, but then it is illegal.
...or did I miss something?
So it is a test of honesty on your part and doing the right thing. Maybe in your country, you've seen the benefits of the taxes especially to those indigent people. But if most of the taxes is in the wallet of some few, then tax has no meaning for me at all. If bitcoin will be taxed, in our country, it will be so sad. Only so few will be benefited by this.