If we want to grow to adulthood as a species and manage to colonize the cosmos, we need to pass _every_ skill challenge. If in 50 years there'll be a risk of unfriendly superintelligence and it'll have needed 40 years of run-up prep work to make safe, then it will do us absolutely zero good to claim that we instead concentrated on risk of military AI and hey, we got this far, right?
Considering the amount of human utility on the line, "one foot short of the goal" is little better than "stumbled at the first hurdle".
I also tend to agree with Maciej that the argument for focusing on the A probability of B is rescued by making the AI threat seem even worse with appeals to human utility like "but what if, instead of simply killing off humanity, they decided to enslave us or keep us alive forever to administer eternal punishment..." either.
But most resources are not spent on risk mitigation. The question of what priority to give risk mitigation naturally (should) go up the more credible existential risks are identified.
Ultimately, AI is neither necessary nor sufficient for the powerful to kill the less powerful.
And if it's powerful people trying to build hard military AI, they probably aren't reading LessWrong to understand how to ensure their AI plays nice anyway.