To play devil's advocate. One of my Steam contacts is actually excited about this for a few reasons.
1. It will encourage more mod support in games.
Something that is a huge problem with modern games is mod support is very sparse. You very rarely see it happen because of how big and complex modern engines are and the costs associated with it. If companies can make a small amount of money from adding mod support they'd be more likely to implement it.
2. It will encourage more advanced modifications
Most mods are broken and forever remain incomplete. This could lead to more polished and completed mods.
However there are a few HUGE downsides to this that I can see.
1. It encourages theft.
This will lead to people stealing mods and selling them. While there is a DMCA policy this still will happen.
2. It has already lead to people taking free mods and selling them.
Wet and Cold was a free mod that was turned into a paid mod.
Midas magic, one of the most popular polished free mods has already become a paid mod I guarantee these will not be the last.
3. It will lead to people making incomplete mods to make more money.
I can imagine people splitting mods up into smaller packages just so they can make more money. Like "oh well you want Falskaar? Well there are now 3 packs for Falskaar all for 5.99$. And if you want the mounts we previously made for it that's an additional 2$. And if you want the extra races we added with it you'll need to pony up an additional 4$." This already occurs with DLC.
4. Mod creators only get 25% of profits
Just what
5. You must make over 100$ to make any money
Bare in mind this is PAYOUT. Not money earned.
Picture this, you sell your mod for 1$ and you only get 25 cents per purchase. You need to make 400 fucking dollars before you see a cent of it.
EDIT: Upon further discovery it seems Bethesda
were the ones who wanted to do this.