I heard from someone who works with people who do phone antenna designs that right now, they gain 15% coverage on the lower frequency, then lose 15% on antenna efficiency problems. My T-Mobile contact said they're not interested in anything lower, having satisfied themselves with 600 MHz.
You're not wrong that it could be used for fixed services, but I don't think the money is there for fixed services. White space devices already exist and are already in use in those bands, and are currently almost exclusively fixed, and there aren't really that many of them because it's hard to manufacture equipment that's largely rural-only in use. I'm not really sure how it would come to pass, considering that in the cities, fixed service isn't necessary because the higher frequencies will work just fine with greater bandwidths, smaller antennas, and at a lower price point, and I'm not sure you can get equipment manufactured for a rural-only user base.