Cash is, and will forever be the tool for tax evaders. Even if Bitcoin became the dominant currency, cash if still in existence would be used by criminals since there's no recorded record of it exchanging hands, unlike Bitcoin.
One of the biggest problems of Bitcoin and all other crypto currencies is the problem of tax evasion. All countries know about the person's assets, but this is not possible when crypto money is involved. If there were no such situations, states would not stay away from bitcoin or other crypto currencies. I don't know how to overcome this.
This isn't true in the slightest. If the governments wanted to brush up on their tax evasion detection via cryptocurrency it would be much easier for them than cash. Cash is literally the way to tax evade if you really wanted too since there's no public ledger. If you tax evade via cryptocurrnecy you're doing it knowingly leaving a trace. The criticism that Bitcoin gets for tax evasion is unjust. Its the people who use it, and cash that are tax evading, not Bitcoin or cash. I declare what I earn with Bitcoin just as I do with fiat, there's no difference since tax has to be declared by the person with the assets or via an accountant of theirs. The government expects that the individual declares it, Bitcoin or cash is not responsible for an individuals actions.
Cryptocurrency while sometimes being a little confusing, and adds a little more complexity to declaring the tax, well at least in the earlier days. Still, is expected to be declared by the government. It's the person who decides to declare it or not.
I've never really understood the need to tax evade especially when your stinking rich already, although I must say the tax over here in the UK at least goes to something I'm quite passionate about e.g the National Health Service (NHS which I like that my tax contributes to keeping it running.