Apple get more than enough through hardware sales of their iPhones.
I think that if we're being honest, nobody here, at EPIC, the Dutch dating apps or whatever is clamouring for "alternate payment systems" if Apple can still arbitrarily stick their commission on top it.
Why do (some) developers want to choose or roll out their own payment and billing systems? To save money. And they won't save much money if Apple can still slap an 11% or 25% commission on top of non-App-Store transactions.
So what could stop Apple from doing just that? More regulation, more action from the competition authorities, more lawsuits - or ultimately, and most effectively, allowing sideloading.