So far, for the last 20 years, modding has been a hobby you do to either please yourself or others. You want to improve something you play and want others to share your enjoyment. You most likely play the game yourself, and you use other mods, so you try to have your mod compatible with other mods and you fix it when an update for the game breaks it.

Let's say though you're not a talented modder, you just wanted to make something cool and it's not great. That's fine, nobody will really care. You made it, some people will enjoy it, other's wont. If it's not compatible with some other popular mod, they will just remove and forget about it, maybe tell you it's not working and you try to fix it, maybe you don't care and stop modding. Someone else from the community might pick it up, work on it, and release it. Same goes for dead mods. Someone else might pick it up and update it.

Now enter paid mods. Sure, some of these talented modders will be attracted to modding now that they can get paid for it, and we might get better, higher quality mods. It will also inevitably attract hordes of people who don't give a shit, but want to make a mod to get money from selling it. Look at greenlight, asset flips and all that kind of stuff to get a little bit of similarity. Then multiply it by 100 because it's far easier to make a mod than it is to make a game.

You buy a $5 mod to improve textures, you like it. 2 months later another mod comes out with better textures that have a lower performance hit, for $10. Now, you simple remove the old one and install the new one. With paid mods, you must spend a $15 total for the same thing. Don't say "oh well you must deal with the fact mods get better with time" if so far you don't have to spend money for them. Don't take something that is free away from the consumers "because I think this is better".

Paid mod is incompatible with another one you're using. The mod developer is not forced to make it compatible, but you already paid for it. I guess you refund it? You paid for it, you expect it to work. Maybe you realise a mod you bought is bad, you need to refund it. Many of us use hundreds, if not thousands of mods over the years they play the game. I assume they should also spend thousands of dollars to get the enjoyment they so far got for free?

A mod actually breaks the game some hours after you installed and played it, as it sometimes happened with Skyrim mods. Do you get a refund after 10 hours of playtime, or you deal with it and make a new save without the mod?

An update to the game comes out and breaks many mods you are using right now. Who is going to fix them? Maybe the developers stopped modding or "phased out" their mods in favour of new ones you have to pay for again? A mod developer gets bored of making mods, or retires for some reason. So far, basically anyone could take a dead/broken mod and update it while giving the original author credit. Who is going to do that now, there will obviously be some rights given to the mod creators to prevent stealing?

TLDR: Paid mods, for every 2 talented, honest modders you will get 50 people whose only objective is to make a crappy mod copied from someone else and sell it for money, not caring if it's broken or not. If you use mods, think of all the problems you ever had with modding. Compatibility, bugfixes, game updates breaking them, finding another mod you like more and replacing it. Paid mods will not fix anything from this list, no matter how hard people claim that they will. If you aren't a modder or don't use mods, why do you speak for those whom this change actually affects??