But they need to open launcher and sign in so they can download editor in the first place 
Seriously if somebody is too lazy to create one more account, he isn’t worth of our time. He won’t create great content if he’s that lazy.
The biggest problem is: if you could simply upload modified editor to Steam, it’s lot easier to set up your own modding scene and provide updates.
Or maybe I’m wrong ? Maybe after initial setup your free to update your community’s editor through launcher without any hassle?