Is it fair to call Apple's platform rent-seeking? I think a 30% cut is certainly far too much but I wouldn't call it rent-seeking. The platform itself provides a lot of value and they are entitled to charge something for it.
It's clear many companies think the 30% cut is too high, but giving up on Apple means giving up on 45% of the highly-lucrative US market.
This means Apple is able to use its dominant position to squeeze the margin of other companies via the too-high 30% fee. A clear monopolistic practice.
As for the value-add (since rent-seeking is about charging without value-add) — it's smaller than you think. Contrast with Android, where you can at least install the app without going through the official App store. Apple's is simply charging for gatekeeping the access to its market (Google would like to do this too but historically hasn't bee in a position to do it as effectively).
The cost to Apple is largely flat. A free app costs as much to host per user as a paid one, and both arguably cost less than a service client like Spotify. And what does Apple add?
I hope the outcome of this is the dissolution of "free" distribution. It's not free, it's sending and revenue to Apple and Google, and the balance is (more than) paid off by paid apps. Flat fees for everyone, pay a cent or so per user per year. It's up to you to make that work.
"Here's the price to have your app in the store, here is the one for one download, here is the one for an update".
Having a blanket "buffet" pricing (sub is 1€ or 100€ ? Doesn't matter it's a set % you owe us !), making it illegal to let the user get the app another way, making it impossible for them to not pay through Apple (and then pay back Apple what you we them for the service you use) is why they're going to lose.
Everything else is just for payment processing, at 10x markup over market rates. Ten times!