So, you agree that Valve should get some part of the sales, because they are the ones (Steam) actually taking the payment and assuming some of the upfront risks of processing payments (fraud) and, because of the network effect and the customers they bring (read ... marketing)
But Bethesda who developed the actual game that brought the modders and gave them a platform to derive their work on, not to mention creating the actual game and game engine and putting it in the hands of the actual customers; they should just get nothing out of this whole deal?
I agree that the whole thing is a big hairy mess, and as a developer and an underdog myself I agree that the split should be approximately reversed, but I'm not sure there's much else we agree on.