I have no evidence to present but it's obvious to me that a subscription to Dropbox, if the payment processing is done by Dropbox, has no added costs to Apple, meaning that 15-30% Apple takes is completely arbitrary.
Now, if the costs associated with auditing apps and their updates were to make up some of that, then that's fine, but there's 1 audit for 1 update, and that update may serve millions of users, so we look at bandwidth and storage instead, both of which are ridiculously cheap. I have a hard time imagining how the charges we're discussing now in any way shape or form can even approach 15-30% per customer, every month...
Yes, but it's in Apples best interest to enable developers to produce free apps, so that's a concession that they are making in all of this.
I think the best solution is for there to be two tiers, the one we have today, and one where the costs are itemised. Apple should evaluate free apps and push them into itemised if they are abusing the free tier.
IAP are an entirely separate issue, if developers want to use Apples system they can obviously do that and everyone is happy, but Apple can not force them to if the developer is willing to build the infrastructure for another payment solution.