His cost estimates depend on hundreds of thousands of people signing up, to diffuse R&D costs. That's dubious.

Historically, colonists are self-interested. Typically they want to become rich, or they want to escape poverty or persecution. (Typically they're also poorly informed about just how dangerous their new life will be, and how much wealth they can expect to find, but in this age we shouldn't expect that to last much beyond the first landing). There's no precedent that I'm aware of for people leaving comfortable, privileged lives in mass to colonize someplace that has no wealth.