Could be interesting, especially in countries like mine where it's incredibly more difficult to find a job as a developper. I don't think it would undermine the work of full game dev but maybe act as a trampoline for more struggling artists.
It really is a giant mess that I admit really have trouble to wrapping my head around.
To reiterate, i really am not against modders charging by the way. (Hell, i go to conventions and sell sometimes some small stuff).
The fact that this is Skyrim really messes things up on a large scale since it's one of the THE most modded game and owes the majority of it's popularity today to the modding scene. And regulating most of these mods, which sometimes use other mods, seems like a total mess outside of valve's ability
Mostly, after a day or two, my biggest gripe is really not against the concept itself of paid mods, but makes me look at Valve and steam with more scrutiny, because I wasn't really on board with their quality control before hand