I don't think we've seen any cyber decapitation strikes, particularly against military systems, but that's exactly the type of action I think many would want to respond to as an outright attack. We have not seen any outright attacks on nuclear powers for obvious reasons. Maybe Stuxnet, but note the goal of that attack was explicitly to prevent Iran from developing nuclear strike capabilities.
By definition you can only respond to a decapitation strike if you haven't been decapitated.
Anyways, I think the nuclear deterrent is an okay argument, I'm not totally convinced by it because I think you could also get right up to the line and even push it without your opponent being crazy enough to push the big red button.
I think we're doing this right now to each other all the time. For example, China is systematically targeting US trade secrets. This seems existentially frightening, but we haven't yet levelled China with nukes.
In fact, if I were a government, I'd secretly have a policy to never push the button, but that's just me.
We put a lot of effort into making sure our nuclear infrastructure isn't vulnerable to a decapitation strike. I have no idea how AI might cripple all subs, for example.
I completely agree that the line is the dangerous part, and it's an unclear target to aim for! I think that's exactly why we don't see major strikes against nuclear powers. Nobody has a public policy stating "we won't nuke you until exactly X, Y, and Z happen" so everyone else is left guessing. This is far from a complete deterrent, which is why we still see probing attacks like Solarwinds, but they do not strike me as decapitation attempts.
That's exactly what I'd expect to happen, but I suspect we may have different definitions of "rapidly" here. Aerospace development requires building and testing stuff, and AI can only help so much there.
Regardless, we're back to MAD doctrines again. How many and how effective are your interceptors? How many nukes do you expect to be launched? What's your tolerance for failure? Margins for error are slim and consequences are rather dire.
We've had cyber hostilities with China for a long time now.
An AI could easily be used in a decapitation strike of military systems.