Nukes are not magic wands, they are mostly only worth it for targeting cities and other very dense concentrations of value and production. An enemy state going down this path will probably start out with cities full of humans (and will probably want to "mine" those cities for materials), but its growing robot production will be more broadly distributed (and EMP shielded) and therefore much harder to destroy. And they will of course have their own nukes. I agree it's possible that a global nuclear war, if it starts early enough, ends with no one still having the ability to operate AI. But that's not exactly the most attractive way to shut it all down, is it?