Depends what you mean by control, because it could be argued that they've already established control by taxing it. If you earn Bitcoin in several countries, you have to declare it as income tax, and I imagine there's a lot of people here on the forum that don't realize that they have to do that. So, technically they have already established a way of controlling it, and could actually introduce harsher tax rates for cryptocurrencies if they wish. That really is the only way of controlling it from their point of view though. They don't have any control on Bitcoin itself, only changing the laws around cryptocurrencies to suit them.
For example, I think they'll look at introducing specific cryptocurrency tax, instead of slapping it in the "tax income" bracket which includes all the income that you have made over a year. Although, I doubt it'll be any much strict that it currently is, and to be honest I don't think they are particularly concerned about Bitcoin at this moment. Its still very much a niche appeal despite its rising price.
Taxes are applicable only if you sell your coins. I can hold my coins until 2030 and pay zero taxes on them, because I am not converting them to cash or purchasing anything with them. So I am not sure when you claim that the governments have some sort of control in the form of taxes. One way to control it may be in the form of wealth tax, which may affect users with large holdings. But reintroducing the wealth tax is a risk that most of the American politicians don't want to take. Obviously there is another proposal to tax "unrealized capital gains", made by Ron Wyden (D-OR). But the proposal is too crazy and no one takes it seriously.