And you charge $9 for it, out of which you keep $6, and Apple/Google keep $3 for the hosting, promotion and infrastructure.
If I develop a similar app, I'll have to compete with you on a similar price point or better features. That's fair competition. We both pay the same fee.
But one day, both Google and Apple both roll out a competitor app with same features as yours (Apple Music), also sold at $9. Now you're in a fix. Not only your biggest competitors don't have to pay a cent in app store fee (they own the store), they also get money off of you.
i.e., Apple/Google get to keep entire $9 plus they get $3 from you. You get $6 only. How are you supposed to keep competing with them at the same price point? For how long is it going to be sustainable, since you're directly financing your biggest competitors?