Funny thing is, I've yet to see a single modder speaking out in favor of this except as a way to stop their work being stolen or because they didn't know what it would entail.
I'd think every modder putting a legit non-joke paid mod on Workshop is in favor of this, unless they're masochists.
I'm looking at the workshop right now. The mods fall into five categories: Wet & Cold, mods whose authors make it explicitly clear that they're still distributing for free (i.e. ones trying to avoid theft), mods which are blatant ripoffs of free mods, mods which are obvious jokes or protests, and "mods" akin to that "shadowscale set" that's a shitty hackjob (see Skyrim thread for relevant links) that some incompetent ass made in an afternoon.
I'm guessing you're counting all of the cash grab shitty weapon and armor mods. Those are mods in the same sense that dropping an extra weapon statblock into DF raws is modding, i.e. minimal effort and no appreciable change to anything. Things like that go ignored on the Nexus for good reason. I'd download to give better documentation of how shit they are, but I'm not shelling out dozens of dollars for the mod equivalent of shovelware.
Arthmoor and Shezrie both have paid mods up and they're both long participating and well respected members of the community. A number of quality mods on Nexus are now appearing on Workshop, either being pulled from Nexus completely, or with the Nexus version being a lesser or outdated version of the mod on Workshop (SkyUI being the most prominent example). Art takes time to make, with 3d modellers regularly being paid on a contractual or commission basis for their work; just because some of the work is shoddy doesn't mean all of it is shoddy. If these people see no benefit to this scheme, why are they setting up shop on Workshop?
One of the better illustrations of how this hurts modders and the community is SKSE: at least half of the major, important mods for Skyrim are dependent on it, and that team isn't going to see shit despite people profiting on the framework they build.
I also find it rather suspicious that they went for a 75% sale on Skyrim the same weekend that this broke.
SKSE was released under the MIT license, which allows others to profit from their work, no questions asked. That's their own doing. If they released under GPL or something similar, then they'd have a case. Regardless, they've already said that the techniques they use are dubious and they don't want money because they don't want the attention of Beth's lawyers.
Steam does sales all the time when there's news about a product in order to drum up interest.
Few reasons. For Cave Story: The guy who made it didn't forfeit his possession of the game, remake, or idea behind it by making Cave Story+. He was taking and remastering his original work, which was free, and selling that. You have reasonable expectation that you're buying a full, working, functional game when you buy it. There's a full product there, there's a value there in the gameplay, in the storytelling, in the art. It's a full experience through and through. Mods don't have that depth, and they don't have commercial support. <snip>
Nehirim, Wyrmstooth, Falskaar, Tamriel Rebuilt, etc. don't have (the potential for) depth? Arissa was an example of a mod that had a basic free version and a paid premium version (which cost the dev sound booth time), so how is that any different from Cave Story? Why are you assuming that all modders will want to monetize their work when running your hobby as a business is a lot more frustrating and work-oriented than running it as a for-fun-only hobby?
Edit: Paid modders on Workshop maintain creative control of their works. If the Cave Story dev decided to give everyone who ever bought the game a refund, he'd get the same response from Valve that Chesko got.