Obvious problems with this:
1) Weaker modding scenes. And no, I'm not just talking Skyrim. The mere existence of an easy way to make paid mods means there will be fewer unpaid mods, fewer modders working on games that do not offer paid mods, and fewer people interested in the modding scene on both sides of the spectrum...
2) ...because the modding scene will become unreliable microtransactions. People are a lot more willing to take risks on free things; having to pay means that they will expect their downloaded content to work properly. This is obviously bad for consumers (patch comes out 25 hours after you buy a mod that completely breaks it? better hope the modder hasn't abandoned it), and only slightly less obviously bad for modders (when a patch comes out that breaks your paid mod, you will know immediately, because every single source of interaction you have with the internet will be filled with hate mail until you fix it).
3) Legal troubles. A company with actually effective vetting might have been able to handle this one, but as Valve has already proven with Greenlight and Early Access, they don't have any interest whatsoever in any sort of curation beyond letting the community handle it. Ignoring the obvious copyright infringement that's going to happen, consider just how many mods are dependent on assets from other mods. When it was all free and Creative Commons and whatnot, this wasn't an issue, because money wasn't involved. Money is involved now. One modder's already taken their mod down at the request of Fore because it used assets from his Idle Animations mod without his permission (permission that he's not willing to grant, as he believes mods should not be for-pay). What if the guys behind SKSE update their license and start DMCAing paid mods that use their code?
4) Activities this incentivizes on the part of publishers:
a) Releasing games as buggy, unfinished messes, to make money from paid QOL fix mods.
b) Sending cease-and-desist letters to third-party mod sites, so their mods cannot compete with the Steam Workshop.
5) Good modders could already make money off their talents, with a donation button on the download page for people who tried the mod and liked it. Great modders could often get hired by the same companies that made the games they modded. And the cut they'd get of those donations is substantially higher than the 25%, $100-minimum that Valve is offering. Pay-what-you-want is nothing new, and already worked. This is 'fixing' something that was never broken for the sake of a quick cash-grab.
Upsides:
1) Valve and the publishers get money they don't even have to work for.
Merely problems that are obvious with a quick glance and a few minutes of thought, from someone who doesn't really care all that much if the mod scenes withers and dies (I don't mod games anyway), and doesn't play or even own Skyrim. Some people will defend this. People also defend day-one DLC, pay-to-win microtransactions, and preorder exclusives. My opinions on such people are one and the same (and usually they're the same people, who like to trot out words like entitlement when, yes, paying money for something does entitle you to certain protections, or at least it used to before they decided to whiteknight publishers who couldn't give a quarter hug about them, or the developers actually making the games, or anything but their own bottom lines).