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.
If you invest in some asset such as mutual funds, then first you need to pay the capital gains tax (which is same in the case of Bitcoin as well). But on top of that, the money you are investing goes to prop up corporations, which pay taxes such as the corporate tax and the labor tax to the government. In case of Bitcoin, this is not applicable. The creator of Bitcoin didn't paid any taxes to the US government.
So if the investors feel that Bitcoin is a better asset when compared to equities, the interest in stocks will go down. The market will crash. As far as the capital gains tax is concerned, there will be no change (since you need to pay the same tax for BTC as well). But if the corporations go down, they can have a negative impact on a lot of domains. For example, the donations to the political parties can take a hit. Board members have most of their annual salary in the form of stock options, and they'll take a hit. Pension funds have a lot of exposure to the stock markets and they'll be forced to book losses. The impact will be widespread. If I have to list out all of them, it is going to take a few hours.
In short, it is in the interest of the government that you purchase equities with inflated P/E ratio, rather than something decentralized, such as BTC.