Hm nvm.
People doesn’t need to pay subscription they need to pay only once to get tools.
Yes 20$ for tools might not be that cheap for someone who is interested only in modding single game, but on other hand if someone is serious about it, is not that much either. It’s one pizza. Maybe half of it. Depends where you live.
You don’t need to expose entire source code, only the part, that you have created and distribute rest on binary form. What have you created can be distributed publicly as far as I know.
Besides it is bit of moot point. Most games do not allow modding for whatever reason. I estimate that people desperate for modding will be willing to even pay for it.