Is this really a fair description of IR Realism?
It’s not a fair description of all IR realism, which I think is a useful theory for illuminating certain interests of state actors. But some proponents (and the article Stephen linked confirms that Mearsheimer is one of them) seem to elevate it to a universal theory, which I don’t think it is. Frankly, Realism is the intellectual home of Russia-apologism, and while I’ll admit that Mearshimer seems to come by this honestly (in that he honestly thinks Russia is likely to respond with nukes and isn’t just a Putin fanboy) I take a rather different view of how things are likely to turn out if we keep backing Ukraine.
Because that kind of countervalue targeting isn’t a thing. I intend to write on this more, but there tends to be a lot of equivocation here between countervalue as “nuclear weapons fired at targets which are not strictly military” and countervalue as “nuclear weapons fired to kill as many civilians as possible”. The first kind absolutely exists, although I find the countervalue framing unhelpful. The second doesn’t in a large-scale exchange, because frankly there’s no world in which you aren’t better off aiming those same weapons at industrial targets. You get a greater effect on the enemy’s ability to make war, and because industrial targets tend to be in cities and have a lot of people around them, you will undoubtedly kill enough civilians to accomplish whatever can be accomplished by killing civilians, and the other side knows it.
The partial exception to this is if you’re North Korea or equivalent, and don’t have enough weapons to make a plausible dent in your opponent’s industry. In that case, deterrence through “we will kill a lot of your civilians” makes sense, but note that the US was pretty safely deterred by 6 weapons, which is way less than discussed here.