Governments can gain even more tax revenue if they don't ban cryptos but regulate them. Just imagine, if they will tax it with a bit higher tax rate compared to the wages and other 'normal' incomes, average people will be happy with it, if they can 'legalize' their bitcoins for a little tax.
And the bitcoin transactions are easily traceable, the only question is if they can link the users to specific addresses. This can be done easily, if someone pays online with bitcoin for some goods and he makes the goods shipped to his address.... also exchanges are forced to implement KYC and AML rules, so goverments should not afraid of the anonimity of bitcoin...