It is changing a system that has been working fine. Modders aren't an oppressed class working without benefit. Modders choose to work on mods for many reasons: fun, practice, boredom, the joy of creating something. And gamers appreciate their contributions. While, some gamers may feel entitled most understand that if a modder is unable to continue the mod may be abandoned. Donations may or may not help but they are an option. This system has for years made PC gaming what it is. Modding in my opinion is the primary benefit of PC gaming over console. Changing a functional system is dangerous and could have unintended consequences.
Now that people are paying for mods they will feel entitled for these mods to continue working. If a free mod breaks and isn't supported that is fine because there is no obligation for it to continue working. If someone pays though they will expect the mod to be updated and continue working as the base game is updated. Furthermore, abandoned but popular mods are often revived by other people; if these mods are paid then the original creator may not want people to profit off of updated versions of their mod.
Related to the above paid mods may reduce cooperative modding. Many mods will borrow elements from other mods; usually with permission. Having paid mods will complicate things. Someone who makes a paid mod will be unlikely to share his/her work with others. What if someone freely share's his/her mod and someone incorporates it into a paid mod? Does the first mod's owner deserve compensation, does the second modder deserve the full revenue. This makes modding more politically complicated and may reduce cooperation.
This may reduce mods based off of copyrighted works. There is a very good chance that any paid mod based off of a copyrighted work will be shutdown. Modders could still release free mods of this nature but it complicates the issue. Many mods based on copyrighted materials borrow (usually with permission) from other mods to add improvements. If these other mods are paid then the original creators likely won't let them use it. Additional many modders may now ignore copyrighted mods in order to make mods that they may profit on.
Steam/the developer are taking an unfairly large portion of the profit. Steam and the Developers are offering nothing new to the situation. Steam is already hosting the mods and the developer already made the game. They now wish to take 75% of all profit from the mod. If the market gets flooded by low-quality paid mods, the modders will likely make very little and the quality of the game will not be increased. However, Steam and the Developers will make money off of no work on there part.
