The main issue for me is quality assurance. Paradox Interactive (presumably) has a business licence. It's run by people who are aware of basic laws regarding the operation of businesses and laws regarding advertising. I feel safe purchasing content with Paradox Interactive's label attached to it.
Modders are benevolent volunteers. They're making content out of the goodness of their hearts, but that changes once modding is monetized. Suddenly those modders have an ethical (and possibly legal) obligation to support their own content. If I make a mod that breaks after Johan drops an official patch on us, how many paying customers are going to be satisfied when I say that I'm retired as an excuse?
Also, please consider the social stress this is going to place on modders. Most people seem oblivious to it, but I've certainly noticed that PDS employees take a lot of abuse around here. Every time something goes wrong, frustrated players attack PDS employee's professionalism, good faith, and sometimes even character. "Johan is balancing this game for multiplayer because he hates us! Wiz dropped a broken patch before Christmas because he is greedy! Groogy won't update the converter because he is lazy!," and so forth. PDS employees are hardened battle veterans of the message boards, and they have access to PR people who can help them if things get out of hand. Modders almost certainly have no experience dealing with a tidal wave of angry customers who feel like they've been cheated out of their money (regardless of whether their complaints are actually justified). I expect that most will buckle or vanish under such pressure.
When PDS breaks something, I have no doubt in my mind that they can fix it. Has anyone considered that modders who may not be the best at modding may not actually know how to fix their mods? Just last week, I was bored, and I downloaded a cute vampire mod for CK2. One of the first things I noticed is that the custom crusade CB was 100% non-functional. I later checked out the topics for it on the workshop and the modder didn't know how to fix it. How long until the Quick Questions/Answers topics in the modding sub-forums are being used for free labour to repair paid content?
In the end, I don't see much coming out of this other than a lot of destroyed reputations.