...and that is the only real issue here. 30% mark up + ~$100/year fee for the privilege of selling your stuff through a high-profile store & distribution chain is a fair deal (buy through a middleman - pay extra - nothing new).
The problem starts when (a) the store owner launches its own competing subscription service which, surprise surprise, undercuts its competitors by the 30% margin; (b) you don't have much control over the price of your service (It's pretty clear that $9.99/£9.99* is the going rate for streaming services and, because ultimately they're all selling the same music from the same rights holders, there isn't much room for downwards manoeuvre) and (c) although you can sell subscriptions directly, the store won't let you advertise this in the app.
The question for regulators is whether there is enough competition in the market to leave this to "the invisible hand" to sort out. After all, the big market is phones, there are more Android phones than iPhones and people change phones regularly, Forcing Spotify et. al. off iOS - or doing anything else to restrict the choice of Apps available in iOS - ought to be a risk for Apple.
Personally, I think the line would be crossed if Apple tried to object to Spotify et. al. emailing customers directly to suggest they subscribe directly rather than via Apple.
My proposed solution would be to set a minimum price for Apps, above which they could link to a 3rd party website for in-app subscription purchases.
(* exchange rate? what exchange rate?)