Most people drop out before doctorates; it's something like 97-99% of the US population. And getting a doctorate in many fields is a terrible idea these days: I looked very hard at continuing on for a doctorate in philosophy, and concluded that even if the grad school offered a fullride, it was still probably a bad idea and almost anything was better.

seems a distinguishing mark of the core SIAI community

Your examples being Will and Eliezer? I didn't realize the core SIAI community was so small.

Is SIAI to serve as poster boy for the libertarian cause of home schooling?

I don't think either Eliezer or Will were much home schooled, nor does the topic come up very much on LessWrong; one would think that Thiel would make his propaganda organs talk about it a little more.