You ignore Valve's intentions. Valve is trying to make money off of mods. In order to achieve this they use their power as store manager. The store front is advertising space and this being used for paid mods is forcing a significant change on the market, which has nothing to do with "allowing" anyone to do anything.
That being said, the change was a long time coming because so far Valve offered store space to modders without getting anything out of it. As they are not a charity this was unlikely to go on forever.
The second misconception is that you think modders are going to choose to monetise their work. Many future "mods" will be designed to be monetised from the ground up. This is unlicensed 3rd party DLC and not a mod. People developing these will not "choose to monetise something they developed" but develop to monetise.
This creates two problems:
Valve cashes in on content produced for a game from another publisher without their consent. Either publishers will cash in on this too (increasing prices and further eroding the "I can do what I want" aspect mods bring to gaming) or even more publishers will prohibit modding from the ground up.
Modders are now either hobbyists or entrepreneurs. As a result hobbyists will be heavily scrutinised as free stuff will be accused to be testing the waters by default. Few modders will be steadfast enough to withstand the temptation of charging for a successful mod. Making something successful and not charging money is a dumb thing to do. Essentially, good mods will become paid mods and there goes your free fun.
The situation is worsened by Valve being shitty curators who cannot be bothered to have some quality assurance in their store. Greenlight and Early Access are filled with barely developed cash ins. The Global Offensive Workshop is bloated with copy right infringements and vote bating shit designed to maybe be lucky enough to make it into the game. Tons of horrible low effort "mods" will be developed for the paid sections. Singularly with the goal to make a quick buck. Valve wants that low effort shit in there because a shitty devs income is Valve's income.
Valve is not a bad company. They are an ordinary company. That's why the deadlock they have on the PC market has to end and this begins with you stopping to defend the anti consumer shit they pull all the time.