Suppose you make the argument that developers, not consumers, pay for the dev kits, language, development, improvement, apis of the os. 30% might appropriately reflect the amount of work in an app that is apples work.

30% of subscriptions on the other hand might not.

Fortunately, Apple breaks it down for us:

Services net sales in 2019 was $46.2B, and the cost of those sales was $16.8B (source 2019 10-K). They dont specifically break out App store vs other services, but as a whole the services section of their business is very, very profitable, according to their accounting. It remains so even if you throw in the entirety of R&D ($16.2B).

Google Play store is also 30%.

> For apps and in-app products offered through Google Play, the service fee is equivalent to 30% of the price. You receive 70% of the payment. The remaining 30% goes to the distribution partner and operating fees. [https://support.google.com/googleplay/android-developer/answ...]

And Steam (unless you have a very popular game) and GOG.

But it's easy to argue that many of these just derived their rates from what existing similar store fees were. Epic Game Store tried to disrupt this and they're still at it a few years later but for most developers publishing on Steam/GOG nothing changed.

OneStore fee is 20%.