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?
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.