macOS app ecosystem isn't really thriving and the regulatory complexities arrived after the Internet matured.

Something being possible isn't the same as being good. Selling 1000 copies at %30 commission(it tends to be around %50 once you ad stuff like VAT) is much better than selling 10 copies at %1 commission.

I also think that from the users perspective it's much better to have one place where you manage all your payments/subscriptions/downloads etc. That can be solved through some kind of unified purchasing interface though.