The claim that AIs will foom, basically, reduces to the claim that the difficulty of making AGI is front-loaded

Yes.

the history of invention in general indicates that the first prototype is a major landmark after which the pace of development speeds up dramatically.

This is not actually true. The history of invention in general indicates that the first prototype accomplishes little, and a great deal of subsequent work needs to be done - even in the case of inventions like machine tools and computers that are used for creating subsequent generations of themselves.