Apple could have a scale where larger developers pay more than smaller ones. As it is now many of the big name developers don’t pay Apple anything (outside of the annual developer fee).
I feel like when the iOS App Store was first introduced, it was operating under the assumption that apps would either be free, paid upfront for a 1-time fee, or ad-supported. In this context, a percentage cut made sense. A high-school kid releasing a free app in his free time wouldn't have to pay a cent regardless of how successful his app may be (imagine something like a ad-free version of Flappy Bird that just happened to go viral). At the same time, it's hard to gauge the financial viability of an ad-supported app because Apple has no insight into those numbers. The only people Apple can bill are developers who release paid apps (more specifically, apps that make use of iTunes billing) because Apple is the one processing those payments.
So to your question, I can see the following concerns:
1) How does one go about billing a company like Facebook or Instagram? The only thing that comes to mind is, ironically, something like the CTF which bills developers if their app installs exceed a certain threshold, but that also runs the risk of unfairly penalising smaller developers of free apps. Facebook would have no problems footing the bill, but you make it more prohibitively expensive for new entrants who may not have the same deep pockets.
2) Size is not necessarily correlated with profit. Should a bank be paying Apple each time their banking app is downloaded, when said app is not directly making the bank any money?
3) I get it may not seem entirely fair, but charging by revenue still seems like the most scalable solution by far. I see how much money each developer brings in, keep 30% (or 15%) and pay out the rest. I don't have to go auditing individual developers or developers either.
4) It's easy and tempting to say that Apple should simply not charge developers for the App Store, but I do also feel that it is not in everyone's best interests for the App Store to be run as a loss-making entity that is being subsidised solely via hardware profits. We already see the current disarray the google play store is in due to Google's apparent lack of care in maintaining it, I can only imagine this is due to it just not being all that profitable, and the end result is a worse experience both for developers and end users.
To use a parallel, Twitch takes 50% of donations from streamers. It sounds rather excessive, until you realise how top-heavy their cost structure is. Someone could be streaming for years to zero followers and raking up server bills while not earning Twitch a single cent because he isn't making any money in subs. As such, it's really the top few streamers who are subsidising everybody else.
However, it is not feasible to charge streamers upfront for the bandwidth they use because then, you raise the barrier for aspiring streamers in an environment where it is common to not make any money for the first couple of months or even years while you slowly build up your viewership.
I don't see either option as being more or less fair than the other. They just come with their respective pros and cons, and there will be winners and losers depending on which stage of their streaming career they are in.
