I'm fairly sure that "motherboard" came about as a term specifically for computer logic boards with slots that other cards -- "daughterboards" -- plugged into. It's very much from the 1970s era when we referred to "microcomputers", "minicomputers" and "mainframes". Granted, I'm a Mac user -- the last time I bought a "motherboard", I think it was a Pentium 4 -- and we tend to use the phrase "logic board" over here in Apple land, probably because, other than debatably the Mac Pro, we haven't had motherboards using the canonical definition for a very long time.

At any rate, while I wouldn't go out of my way to squelch the word, I wouldn't go out of my way to insist on it, either. "Logic board" and "mainboard" both work and get the point across.

Presumably, that would be Main Earth.

Well, not at this rate. Hope our sub earths are habitual in the next century or 2.

There's only one earth so far, so that wouldn't make a lot of sense.