It's insanely hard to tax a currency you can't seize.
They can trace the transactions and possibly find out which address belongs to whom, but actually forcing them to pay up will be next to impossible.
Of course they can threaten with jail, but they'd have to prove they know which owner the wallet belongs to, and to prove it they have to admit they have been spying on us on a large scale and that may trigger revolts.
Also, if taxes are too high and the government refuses to do things for society (like they are doing now already, increase taxes all around and cutting spending all around at the same time) they might not be able to find enough co-operation to even finance the police and/or the army anymore. And then they will have nothing on us anymore.
Those are very interesting points you have made on multiple levels.
However I think you are underestimating. None of us possess bitcoins or any other crypto currency unlike cash or gold can hold in your hand. We do not own anything, we simply have keys that let us use bitcoins, everything is in the blockchain.
We already know NSA has been spying on everyone for years. They know who I am through my IP address.
So imagine this scenario: Bitcoin becomes centralized, which it clearly can be and pretty easily, ie > 51%, something catastrophic happens, state steps up and says we need to take control of this monster or offers alternative and takes over blockchain for "the good of the people." Now can impose forfeiture and will all be voluntary. Rescue me.
Paragraph above pure speculation.
However, I do think ultimate intent is to take control of blockchain. How this will be done I do not know and can only speculate.
This i am convinced of, states want to move society to blockchain. Surrender all assets to the blockchain. This will enable consumption tax, asset tracking etc. Logical step before hand would be taking full control of blockchain which will give society (the masses) total confidence in blockchain aka the state.
I think this is massive social engineering to get people to voluntarily abandon debt based fiat currency and give states more power and control.
Carrot and stick. Bitcoin and blockchain will enable X but also enable states to do Y.
I also think this is a long term project...