As for the 25% to the author of the mod, that does seem unfair, but I don't think that was Valve's decision. I think they left that up to the game producer, Bethesda in this case. Bethesda chose how to split the remaining 70%.
As an aside, Valve's income per employee is totally irrelevant. Their 30% fee is set by market forces. If they lost billions of dollars this year or if they hired 10,000 people to handle support calls, the "fairness" of a 30% fee is not affected. It wouldn't become more fair just because their income per employee ratio changed.
That 30% is set by market forces. It's what we've all generally agreed a marketplace can charge for operating the infrastructure. In this case, Valve has a challenge, because there are multiple parties splitting the remaining 70%. So they might need to lower their fee to let the game producers and the modders take larger slices of the pie.
