ICBM's remain unused due to clear mutually-assured destruction. Autonomous weapons are much easier to introduce to actual warfare and once introduced are assured of continual improvement until...what?
Whatever we think the limits are, we're probably wrong.
Many things. What matters is real world outcomes, not hypothetical potential for destruction. The US doesn't use ICBMs, so in the real world they are effectively harmless. The US public and the rest of the world will not allow their use. Autonomous mechanical soldiers, on the other hand, may very well be fair game.
Okay, compare the muslim/"Mughal" invasion of India to oh, say 100 Hiroshima's. Then tell me which is worse. I think you'll agree that the invasion was much, much worse.
ICBMs are not the world-enders their reputation seems to indicate. The real problem with ICBMs is that nothing can stop them. You know where someone is, an ICBM can kill them, with very, very few exceptions. No other technology will do that. That's why they're so feared. Not because they'll "end" a country, but because politicians fear for their own skin. Russia can destroy the white house, and there isn't a damn thing the US can do to stop them using ICBMs (one would not reliably do it).
By far the most scary weapon in the world is extremely humble : knives. Not even "scary" knives. Think blunt, 10cm long or less, knives. They are by a ridiculous margin, the weapon most people were killed with. The second place are short, curved swords.
Whatever we think the limits are, we're probably wrong.