But surely that is precisely because Bitcoin hasn't really taken off so far --- at least not as currency per se (although it kind of has, as investment, or at least as super-volatile investment aka speculation)?
Had people actually started using Bitcoin as currency, then absolutely, Russia could indeed have taken advantage of it. And if people do adopt it as currency in future, then in future imposing sanctions will become that much more difficult, maybe even impossible.
Which is kind of the whole point of cryptos, isn't it? Not let the government get on your case. Not when it's seen as intrusive and undesirable; but also not when it's seen as necessary and/or desirable. I don't think it's even possible to make a government-free currency that's government-free only when "we" want it, and otherwise not, not even in theory.
So, I mean, this can be seen as an argument against cryptos, or it can be seen as an argument in favor of them, depending on who/where you are, and also how you see this whole business of government control.