> And when I reach the part where the AI, having copied itself all over the Internet and built robot factories, then invents and releases self-replicating nanotechnology that gobbles the surface of the earth in hours or days, a large part of me still screams out that there must be practical bottlenecks that haven’t been entirely accounted for here.

This is the crux of the issue. There's simply no clearly articulated doom scenarios that don't involve massive leaps in capabilities that are explained away by the 'singularity' being essentially magic. The entire approach is a doomed version of deus ex machina.

It also appears quite telling the traditional approach is focused on exotic technologies, such as nanotech, and not ICBMs. That's also magical thinking.