I think it is not realistic to be deaf in the country for every citizen, how much he owns crypto-currencies. Yes, and still recover the tax from it. To be able to collect taxes, the state should control the system itself, where the crypto currency is actually used - and this is not realistic for a single country. Scales are too huge.
In my own opinion, taxes should not be applied on cryptocurrency investors because they are already paying a decent amount such as transaction fee's that will compensate as a tax.