Without stringent quality-curation, this is an awful idea.
Strike that, this is simply an awful idea.
Free enterprise works because product has to overcome overhead to keep producing product. Businesses can fail. Under this model, there is no incentive for junk modders not to flood the Workshop with garbage. Without any warranty of recourse, people will also rapidly become unwilling to pay for it. Finally, there's the matter of how mods frequently share credited assets, and this just Balkanizes the community.
It's a terrible, awful, tragic idea. I think the modding community can be rewarded for its efforts, and game developers have been recruiting from modding communities for ages. I don't dislike the idea of a tip jar - an optional donation feature that splits in a similar distribution formula if someone wants to chip in to a job well done, but leaving this up to mod authors is horrifying. I don't tend to use many mods as it is, but this is a sure-fire way that I will pretty much quit altogether without some kind of quality control.
What the **** was Valve thinking?