As we begin seeing robots/computers that are more human-like, people will take the possibility of AGIs getting out of control more seriously. These things will be major news stories worldwide, people will hold natural-security summits about them, etc. I would assume the US military is already looking into this topic at least a little bit behind closed doors.
There will probably be lots of not-quite-superhuman AIs / AGIs that cause havoc along the road to the first superhuman ones. Yes, it's possible that FOOM will take us from roughly a level like where we are now to superhuman AGI in a matter of days, but this scenario seems relatively unlikely to me, so any leverage you want to make on it has to be multiplied by that small probability of it happening.
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BTW, I'm curious to hear more about the mechanics of your scenario. The AGI hacks itself onto every (Internet-connected) computer in the world. Then what? Presumably this wouldn't cause extinction, just a lot of chaos and maybe years' worth of setback to the economy? Maybe it would increase chances of nuclear war, especially if the AGI could infect nuclear-warhead-related computer systems.
This could be an example of the non-extinction-level AGI disasters that I was referring to. Let me know if there are more ways in which it might cause total extinction, though.
In a recent essay, Brian Tomasik argues that meme-spreading has higher expected utility than x-risk reduction. His analysis assumes a classical utilitarian ethic, but it may be generalizable to other value systems. Here's the summary: