It's probably mainly a matter of personal opinion, but while I agree we should find a way to encourage better and more professional modding, I think that this particular implementation would create a technical/legal/ethical mess out of something which has worked well for the past decades. In short, they half-assed it, and it being one of the first implementations of this kind of system (and thus setting a huge precedent for other games) it should absolutely
not
be half-assed.
The particular problems I have are:
* Developers are already shipping less and less finished games knowing that they can pile on some more content afterwards in DLCs, for a steep fee. Knowing that they can just provide modding tools and let the users create the content themselves and still get a huge part (45% in the case of Bethesda) will just encourage the release of even more broken games.
* That's not even considering the fact that it encourages modding to go from a relatively open thing (you can open any mod in the mod editor software which you can get for free and add stuff to it yourself, and even redistribute it with the permission of the author) to a closed thing: there would be a big incentive to tack on some DRM to stop other people from "stealing content" from paid mods and create their own.
* Finally, it would turn the Steam workshop marketplace, which is already a mess, into the same thing that different app stores have become: a complete mess filled with garbage copycats trying to con people into giving them a few bucks.
I do not have a solution, but I think there should be more thought put into it. For the moment, patronage style remuneration and crowdfunding are the best solutions we have.