I like your thinking on a frame that can work with a few wheel sizes. I've built 650b wheels for my newest Heckler, like them a lot. Don't see a concern with parts availability, though. Sure, 26ers and 29ers there's certainly more choice in tires and rims, but there's enough choice to start and it's growing; the 650b wheels work with quite a few 26er frames and forks in any case (and not often true for 29ers, especially forks).

ChrisF I think has it right about why a lot of people pass over steel, between the weight weenie thing and the pricepoint/material cost thing, which is a shame. I've also had some people express concern over corrosion. While I think aluminum is a marvelous choice for a full suspension frame, I don't see it being superior to steel in a hardtail. I haven't ridden an aluminum hardtail that compares to steel (supposedly there are some aluminum alloys that approach it, but haven't had the opportunity to try any of those). I grew up on steel frames, so I'm prejudiced somewhat.