It will be foolish to believe that the government will just sit back and watch the show, when the national currencies take the hit. The tax revenues are going to get affected and the entire apparatus will feel the impact. So at some point, they will be forced to initiate action against the cryptocurrencies.
Can you explain me how the tax revenues as you call it will be affected? I for example can choose to not trade and not profit, which means no tax going towards the government, or I trade and profit but not pay any tax. So how exactly will the tax revenues be affected? The government can't lose or be impacted by something it never had in the first place.
I personally do pay tax over the profits I make because it's done through centralized exchanges, but this is pure profit to them. It's money they have now that they didn't have before, and didn't count on to generate. Governments have to work on being competitive and fair so that crypto businesses will choose their country to settle in. It's a new (additional) source of income for them.