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?
I mean... literally everyone is doing something illegal one way or another. People do drugs during the weekends yet the only people that get caught are the dumb ones and the big fishes. If you stay low key and know what you're doing, you can never be caught. Just like you said, you can hide some money under your bed and you will not be caught. The same thing can be done with bitcoins.
People will start hiding their bitcoins on different places like exchanges, trading sites, mixers, gambling sites. But if they are lazy, they are just simply going to not declare the bitcoin addresses. After all, you can create a million of it. One thing that people will surely do is declare a couple of bitcoin addresses and make that seem legit by having a transaction every now and then. It would be doing an illegal stuff but not giving your 100% of doing it. You're going to declare taxes but not all of it. That's what people would do.
Another way to avoid this taxation is by selling your bitcoin by meeting up with another person instead of them sending the money through the banks.
Yep, you might have found an easy way to put tax on bitcoin but people are smart. "You found an easy way? Well, here's an easy way around."