It's a disaster waiting to happen for several reasons.
1. Valve/Bethesda is charging the mod creator with a 75% cut of their profit. You thought Nintendo's 30-40% creator plan was bad, Valve's idea just made Nintendo's look fair in comparison. To make matters even worse, you're only able to make that 25% profit on your product if you pass the 400$ mark. Meaning that if you fail to reach that mark and quit, you get none of that money while Valve gets all of it in turn.
2. Remember how we despise DLC's being used to nickel and dime you for small items or upgrades to your inventory? Well now mods may suffer this in turn with the flat paywall they're giving to modders on steamworks now.
3. This may discourage modders from sharing their mods with other mod creators unless they form a team of sorts. One of the best things of mods is that the modding community is generally friendly with each other to the point that they would often go and share their mods to other modders for them to use. Now it may turn into an exclusive circle among friends where if you're not a friend to the modder, his group, or aren't willing to share a bit of the 25% of the profit you only earn from the mod you want to create, then you won't get that mod in question.
4. Valve is only offering you one recourse for a refund on a mod. That recourse is a 24 hour period where you can demand a refund for the mod. Now the issue lies in this.
A. Say you found a collection of mods you like and are enjoying it. Weeks later you found another mod but the issue is is that it isn't compatible with the mods you have. This means that the only way you can play that mod would be to uninstall, which can be an issue if you're not knowledgeable enough on how to uninstall it, the other mods that you're going to need to shelve until you decide to use it again lest you waste whatever amount of money you spend on it.
B. This would scare consumers from getting mods. While year old games like Skyrim are most likely safe from a patch, however what if there is a patch the developer released that not only ruins the mod, but the mod creator cannot find a way to make his mod work? Yep, you just wasted money on the deal. This is actually even worse then paid dlc in games as at least you know developers have an obligation of sorts to ensure your dlc works as intended. The modders will have no control over if their mod can work or not and what if the modder decides to no longer update his mod anymore. You're playing Russian roulette with a paid mod to see if it will work in the future or not due to a patch and if the modder is willing to work on it or not.
C. It defeats the whole purpose of modding in that you're modding the crap out of your games to get the best results. With this paid mod scene that only offers you one small window to save your money in case it doesn't work, will you be more or less encouraged to find mods if such a flimsy protection was available?
5. This puts a bigger target crosshair on modders who are using Valve's new paywall system for mods by content creators and companies. One of the biggest reasons why mods have been free previously was out of safety rather then out of want as TB makes a good point that modders should receive monetary compensation for their work though not in this manner. Content creators who are protective about their IP are less likely to go after you if you make it for free and without intent of making a direct profit off of their product as it's not harming their bottom line, and their copyright is relatively protected under the whole 'free advertisement' thing they can claim in this very grey area. Now if they hear a modder is making a mod out of their products image and making a profit out of it, they're far more likely going to send a C&D to that modder to protect their copyright. Thus this may in turn actually discourage modders and limit the scope of their creativity as sure game developers like Bethesda may approve you making mods in their game, but if a company like Viacom finds out you're making a star trek mod on that game, it won't matter if Bethesda owns that game, they'll C&D your mod for violating their copyright.
6. Lazy and greedy modders may also start churning out mods that are clearly unfinished or poorly made purely for profit. This makes your purchase of a mod even more risky as what if that mod happens to be a poorly made one?
Honestly this is just a very greedy cash grab attempt by Valve to corner the modding market to their needs, and Bethesda in trying to profit off of the immense size of the Skyrim modding community as much as they can. I wouldn't mind supporting modders through a patreon like system, but not through a paywall that Valve is doing instead.