There's some fundamental stuff that limits Linux to fairly complex CPU's (@vertigo could explain this in more detail than I can). This makes it a lot harder to use on synthesized CPUs, which have a lot more potential for being open.
If you didn't have to support Linux, and you didn't have to support the "modern web", you could make computers a lot less complex, a lot more energy efficient and a lot more open.
Also there's Unix architecture things that I don't think are necessary for a personal computer, for example all the stuff that goes into multi-user/timesharing.
