There is no per seat / per developer / per engineer fee. "A developer" in this context means an individual person, not an entire organisation.
An organisation pays $99/yr to publish on the App Store. That covers all their developers, whether they have one or a hundred.
In addition to that, you don’t have to pay anything to be a registered developer. It’s free to register as a developer. You pay the fee to publish on the App Store.
Apple having "20 million registered developers" doesn’t translate into the vast sum of money you seem to think it does. The income comes from the number of organisations publishing on the App Store, which is surely far lower than 20 million considering there’s an order of magnitude fewer apps in the App Store than that and publishers can publish many more than a single app with their account.
For a popular income generating app, the $99 will make an insignificant impact on the marginal cost. But the vast vast majority of apps are not in that category, and the $99 does have an impact.
I often wonder how they count those developers numbers if it was not paid Apple Developer programme.
It’s not about Apple being anyone’s friends. It’s about being logical.
Apple has 20 million registered developers.
[Edit: And let's say a third of them pay that fee, which is admittedly probably a stretch. The finer details are not my point here other than to say here's a bunch of money that comes in no matter what]
I don't care what those developers are making: that's serious income right there. If Apple think they are entitled to more, that's their problem, but don't act like they're making any kind of sacrifice here, and for fuck's sake people need to stop trying to convince themselves that Apple are their friends.