Could Apple then argue that they aren't the only game in town because of things like subscriptions and web apps? (e.g. I don't pay for Netflix or Spotify through Apple, but the apps work fine)
No. Monopsony and monopoly don't work like that.
See eg https://en.wikipedia.org/wiki/Monopoly_price
Basically, when a monopolists or monopsonists tries to abuse their position, they will shift less volume. There's a finite optimal cut that maximizes profits.
So: when Apple increases their cut, they'll get fewer apps developed. Marginal app developers will go work on Wall Street or flip burgers etc.
If a customer signs up and setups payment for the service outside the app, then Apple doesn't take a cut. That's why you can't buy books in the Audible iOS app, or subscribe to Netflix via the iOS app. But you can login and watch/listen to content you've already bought or subscribed to.
In the past, developers were prohibited from telling users that they could sign up cheaper on your website, allegedly to avoid user confusion; I think it was relaxed somewhat so I don't know if it's still the case.
Of course the main reason why they charge 30% for the first year is because they can, and the reason they charge 15% after that is because app/content developers have been starting to gripe about it (probably after realizing they need to make a profit and can't live entirely off VC money). It has little to no relation to how much it actually costs Apple. There's no justification for why it takes a year to get the reduced rate, instead of something like 2 months.
I really don't get why everyone is bashing Apple for this like everyone else is doing the same thing. Yes, we can argue about openness of Google Play Store, but it's also full of malware, crapware, shitware and other BADware. ;)
Furthermore e.g. Apple does compete with Amazon kindle store on the Apple platform, but Google doesn't compete with AMamzon yet imposes almost identical terms on the Amamzon kindle store on Android. So if it's ok for Google to apply those terms on Android, why is it not ok for Apple to apply those terms on iOS?
This whole line of reasoning simply doesn't make a lick of sense. If it is reasonable for Apple to charge companies on their platform at all, then the evidence in the market is that their current charges are competitive and reasonable.
There was that story last year about them ranking their own apps above competing 3rd party apps when searching.
https://www.wsj.com/articles/apple-dominates-app-store-searc...
Someone needs to sue Apple for this, and the truth will be revealed when the case enters discovery mode, when Apple internal communications on this are laid bare
For example, Apple prioritized service workers over web app manifests (which BTW are not yet a W3C recommendation). I can only assume this is partly because there have been so many failed attempts at static web app manifests in the past. Them prioritizing other features for earlier release doesn't mean that Web App Manifest support isn't in active development ( https://bugs.webkit.org/show_bug.cgi?id=158205 ).
1. Binary WebRtc data channel messages
2. Browser notifications that a PWA can use
3. Web App storage beyond 50MB, and guarantee that the OS won't whimsically delete storage, despite being below 50 MB
4. You've already cited Web App Manifest
There's probably a ton of other W3C standards not in compliance, but the above list is pretty damning.