I want to argue several of your inaccurate points
Feel free. I never post anything unless I am prepared to back it up. The only thing I won't do is tolerate down votes.
What I WILL inform you of however is that Valve doesn't take 75% in any way.
Yes, they do. If you make a mod purchase via Steam, the revenue sharing between content platform and mod creator is 75% / 25% respectively. The revenue sharing split between content platform and game publisher is 30% / 45% leaving Valve with a final 30% as the content platform.
As for paying Bethesda again for what you already bought. You aren't.
That is the same tricky logic I've heard to justify the opposition to network neutrality. Ultimately, it is coming out of my pocket. If I pay Comcast $49.99 per month, I should have unrestricted access to the Internet. Comcast agrees, but they also think they should be able to charge Netflix for the traffic sent over the network to me. As I pay for Netflix, the money paid to Comcast will be paid indirectly by me as part of my Netflix subscription. To summarize it simply, I would be paying for my internet connection as well as the traffic sent over the network to me. That is wrong which is why we have network neutrality.
If I buy Skyrim for $49.99 when it comes out, I have paid Bethesda for the game. The resources licensed from it by modders are those I have already paid for in full at retail price. When they charge the mod maker 45% of the purchase price I pay, that is money I am paying to Bethesda indirectly via the mod maker for resources I have already paid for at retail price. I don't have a problem paying Steam their cut as the content platform facilitating the sale. I do have a problem paying Bethesda the 45% when I have already paid for those resources at full retail price. Double charging me, even indirectly, is bullshit.
making it significantly EASIER for people to get into modding
The way most mod makers get into the scene is to learn from existing mods, existing released resources, and so on. Those resources aren't put out by Behtesda. They are created, previously for free, by the community itself. As we are in the era of "Fuck you. Pay me.", those resources may not be as available as mod authors attempt to profit from their work instead of releasing material that creates competition. You have to remember this isn't just about Skyrim. This will ripple out into every game yet to be released making modding less community driven and more profit driven from now on. Free resources don't exist in the world of "Fuck you. Pay me" because why should the people writing them work for free. They have just as much right to say, "Fuck you. Pay me." as anybody.