I think the embedded wafer level or "panel" level packaging technologies are the mid-ground. These technologies don't use expensive silicon, and instead surround the die with cheaper epoxy. Then the interconnects are built on top of that, and can connect multiple die together. Yield and interconnect pitch are the big issues here though, and that's why I think you're right, that we will see SoCs or mobile systems first, not whole motherboards.

With that said, some of these technologies can have a layer of surface mount pads on top. So you have a substrate of epoxy with all your chips and interconnects embedded in it, and then surface mount parts on top. For example, passives, connectors, etc. It would look almost like a motherboard, but with all the chips inside. Of course, for cost and yield reasons, this will be for mobile devices only at first.