Also, if you have rocketry that's efficient and high-tech enough to push distant space rocks into deadly orbits, building a good ICBM arsenal is almost the same thing. Although dropping rocks at 50km/sec does save you having to build nuclear warheads, and you'd worry less about them getting shot down by missile defense.
But global-scale strategic deterrents do get kind of academic past a certain point. There are plenty of ways for a nation-state actor to end the world if they really want to.
I'm thinking a few decades in the future, when there is probably settlements on the Moon and Mars, and an asteroid mining industry that finds valuable metal asteroids and sends them to Earth orbit, maybe by attaching solar powered rockets to them.
In that world, any asteroid mining operation can destroy a lot on Earth, even by accident.
But global-scale strategic deterrents do get kind of academic past a certain point. There are plenty of ways for a nation-state actor to end the world if they really want to.