BW -- I think Byron is looking for an 80/20 bike, as in 80% asphalt / 20% gravel rail trails and smooth doubletrack. I still maintain that a 29er is not the right bike for this kind of mix, but a CX bike would be just right.
That said, I agree with what you say. For riders tall enough, the 29er size is the ticket for a mtn. bike. Very versatile and capable for both singletrack and flat gravel trails. I don't think the 29er will ever replace the 26er though. Smaller riders will just not ever be comfortable or properly proportioned on a 29er bike, no matter what they do with frame design.
But.....what is a really interesting development, and something I look forward too seeing growth in, is the 650B wheel size. Now that is a size that makes sense to supplant the 26 inch wheel. It has many of the advantages of both the 29ers and 26ers, and almost none of the drawbacks of either. I have read some comparison tests, and it seems to work. Nice. I could see a smaller rider, like my wife at 5'7", for example, having a great time on a 650B hardtail XC bike. I hope they catch on and we see more models and aftermarket components become available.