>A desktop motherboard, to me, includes things like audio and video support
Ideally, what most of us want, I think, is a ARM ATX motherboard, with a few PCI Express slots. I believe that it would limit the selection of processor though.
Yes, exactly that. The typical PC motherboards start at $100, and have so since the 90ies. Just one of those, with a socket for their cpu and a translated bios just good enough to bootstrap Linux should not be so much more expensive. Even if they had to sell it at cost it would be worth spreading the technology.
They are, however, mass-manufactured, and how to build an Intel motherboard is well-known, there's very little research cost involved. An ARM motherboard really couldn't be mass-manufactured - there wouldn't be enough demand - and you're starting almost from scratch in terms of knowledge of how it integrates with the processor.
Also, if you wanted a socket, you'd have to actually design a socket for your ARM chip, and then sell ARM chips that are socket-able, with few people who actually want them.
Ideally, what most of us want, I think, is a ARM ATX motherboard, with a few PCI Express slots. I believe that it would limit the selection of processor though.