They have to be "compensated" in the intellectual property[0] sense of "we reserve the right to invent new reasons why we need to be compensated". Nothing is ever truly "paid for" or "owned" here.

[0] "Federal contempt of business model"

I’m not sure what % of their R&D budget comes from the $99 fee vs various other AppStore percentage based fees. But… should it be a flat fee? It seems sort of reasonable to charge more successful apps more, they are apparently benefiting more from the ecosystem, right? Like progressive taxation. (If anything, why not institute increasing developer “apple tax” brackets?)

It looks like, just from some random googling, Apple makes somewhere in the range of $85B per year from their App Store, and there are around 34 Million iOS app developers. Do people really want to pay north of $2000 for their developer licenses?

I think you'd be hard pressed to take the $99/yr they make from the dev program fee and use it to cover the salaries for the engineers implementing and maintaining all of iOS' developer-facing APIs.

Who decided that developers should be the ones paying for the development of those APIs in the first place? Are we just going to ignore Apple's own products and services that their platform allows them to profit off of? And the market share afforded to them by supporting popular third-party apps and services?

There's plenty of precedence for platforms being profitable even with free APIs - including Android, Windows, and even Apple's own MacOS. iOS is not special.

Apple would pay for those APIs whether or not the dev program fees alone were enough to cover the expenses. But they'll also take as much from the devs as they are legally allowed to. And if the fees are enough to keep devs from distributing outside the app store, even better for Apple.

Isn't having good apps/api a selling point for apple hardware (where they already make massive amount of money), why can't that be a motivation by itself?

Why would that fee pay for all that? Why wouldn’t revenue from sales of iPhones pay for that?

Of course. But they could just raise that fee.

and what about the devices itself? doesn't apple get money from selling iphones and ipads?

The only downside I see on the DMA is that it has come very late, and that it's only an european law. Mobile devices are computers, and once sold you should be able to install whatever you want like on any other computer. The shame on apple is that it is increasingly difficult to install software even on the computers.