I think this will greatly decrease the prices of cryptos.
How so? If they decide to tax it then all people from those countries will dump their crypto and run to... fiat? Fiat is taxable as well. Gold? Can't hide it so easily. Houses in Somalia? Too risky...
These governments will never know that individuals from those countries have crypto until they say so. Or possibly if they trade on exchanges in those countries (technically no, but I wouldn't be surprised if the tax auditor came after you with records of all trades). Or bank activity. Any deposit into an account in any of those countries is reported instantly, no matter where it comes from.