I didn't have mice or keyboards in mind, but yeah, a full stack would be surely overkill for their minuscule uCs; this would call for more intelligent devices, peers rather than slaves, which for smaller ones would be a problem now. I'm far from an expert and you're 100% right on the involved complexity, still I dream of a standard for devices and systems where just adding a software layer can extend a board to board communication to a geographic level, and Ethernet seems very close to that.