Isn’t Apple’s user base pretty much anyone who has access to the internet? Should ISPs and cellular providers also get a cut?
It’s a lot more nuanced than that.
Apple’s value-add here is the provision of an App Store that
1) increases people’s confidence in the safety and quality of the apps located inside
2) makes it easier to make purchases (iTunes, Face ID / Touch ID authentication). Lower incidence of piracy means more people purchasing apps from you (and actually paying for them), rather than downloading APKs for them.
3) Apple also provides a lot of SDKs that improve the functionality of iOS apps, making them work better for the end user.
4) Apple has also aggregated the best spenders. My ISP doesn’t provide any of that extra value-add.
All this makes iOS customers more open to purchasing apps in the App Store. Therefore helping to sell more apps compared to if the App Store hadn’t exist, and customers had to download apps from various websites the way we do on macOS.
What Apple has done here is help grow the overall pie. We can debate whether that’s worth 30%, but my point is that Apple is doing a fair bit to facilitate that transaction between the developer and the end user, and they are not just collecting a commission for nothing. Apple is actually providing something of value here.
This also means that operating an App Store also costs a lot of money, which Apple offsets using the revenue it collects. It’s not reasonable to expect that Apple absorbs all the costs of doing so (it would be like expecting a departmental store to display your goods for free, at zero benefit to itself), and $99 per developer per year only goes so far in covering the costs incurred.