Something occurred to me while reviewing some of Paradox Interactive's terrible mod policies that I think may be an additional argument against paid mods.
Companies are currently able to regulate the content of mods (or add rules for their distribution) within services that they own or have authority over, like a forum, website, client, the steam workshop, etc, but at least on PC there exist external places for people to share and distribute mods that companies do not exert control over. Third parties may issue a cease and desist to these sites over copyright violations, but generally the company owning the game will never consider it worthwhile to police the content of mods distributed elsewhere that violate their "home rules".
If that company were selling paid mods, however, they would suddenly have a need to police people taking the very easy step of copying a paid mod and distributing it for free somewhere else, something far easier for people to do than illegally distributing a DLC, for example. So an almost necessary subsequent step of a company selling paid mods (if it ever makes serious revenue) is also putting in place resources and a protocol towards policing the whole public internet where the mods may be distributed, like they already do for piracy. If they do this, the inordinate amount of control they already claim but fail to enforce over the content of mods could then be applied anywhere.
Paradox Interactive, for example, forbids modders accepting donations, forbids using or sharing any kind of license for their work or portions of it (such as music), and forbids distributing the mod outside their approved services, but are barely willing to enforce this even within their own services while there isn't any money in it (I've personally broken the license and distribution rules for years now with no problem). They even for many years forbade modders from using an external public forum to discuss and develop their mod in, as if they have any control or legal basis for that. If there were money in it, then any hypothetical company could get away with enforcing whatever silly or even illegal rules they want by throwing C&Ds around on third party distributors. The companies already often claim that they own the copyright on any mods created entirely, they could shut mods down for whatever reason they like if they feel it's impinging on profits (real or imagined).
Just an additional point for why paid mods may introduce unpleasantness.