Probably very little, since artists are terrible at getting paid in general 😂

And I agree, even 5% might be much without the bigger artists. But there are a lot of indie producers, and I really don't see why they would publish elsewhere. We should expect that most indie artists on for instance Soundcloud, will come over after some time.

Indie labels accounted for 6 billion, according to this article: https://www.billboard.com/articles/business/8006977/indie-record-labels-global-market-share-2016-win-report , so that might be somewhat of an indication. And Choon will allow smaller labels from next year.
But it actually sounds like a lot more than I had thought. But yeah, I guess that is the potential total market before the big guns are in, excluding the other features that will increase demand.