I think there's a lot of nuance in this situation.
First: the most vocal developers really do not care about Apple's "ecosystem invented out of thin air". They just need to be where their customers are, wherever their customers are. The Spotify's, the Google's, the Netflix's, etc. The users who leverage the iPhone to access their Spotify subscription are Spotify customers, not only Apple customers. My relationship with Spotify pre-dates my relationship with Apple. I signed up for Paramount Plus due to a Superbowl Ad, not due to some contorted funnel which Apple, believing themselves to be the center of the universe, have inserted themselves into.
If I could download these applications directly from the suppliers: I would. And, in fact, I do so, on Windows, on Linux, and even on Apple's own MacOS.
The cognitive dissonance I can't reconcile among advocates for Apple's side is: simultaneously saying "Apple built this ecosystem, they deserve to tax it however they will" and "If you don't like it just go to Android". Ignoring the obvious problem that a duopoly is only marginally better than a monopoly: one of these has to be a weird argument, right? Either the ecosystems are not so-fungible as to be interchangeable for the purposes of marketplace competition; or, what Apple created isn't unique, it is fungible, and they shouldn't get as much credit as they have.
I trend toward the latter; I don't believe what Apple has created is all that special. I think their App Review does vanishingly little to improve the functional security of their end-users. Their software technology is almost universally crappy, especially when speaking on anything developers have to interface with. The next few years will prove even their hardware to be un-special, if it isn't already. They've built what they've built on the shoulders of giants. AT&T built a nationwide fiber network "out of thin air". Verizon built a nationwide 5G UWB network "out of thin air". TSMC builds state-of-the-art chips "out of thin air". The open source community built FreeBSD "out of thin air". Slack and Spotify built their application experiences "out of thin air". If even one of these entities didn't exist, or made the decision to stop dealing with Apple's unique ecosystem, Apple would face an existential risk. If Slack wasn't on the iPhone: I wouldn't buy an iPhone. If iPhones did not work with Verizon: I would not buy an iPhone. The list goes on.
Just think on that for a second: should AT&T have the right to say "our fiber backbone won't carry traffic generated by iPhone endpoint devices"? "Ultimately Apple is gaining unfairly access to our customers". "They represent a security threat to our customers". That would be insane! If its even possible, it would literally overnight destroy Apple as a company. So then it becomes, where do you draw the line? Why is it "obviously not ok" for AT&T to do that to Apple, but "eh its complicated" for Apple to do that to Epic Games?
Your argument is that this is an Ayn Randian situation where Apple put in the work to win over these customers, and now devs want a free lunch. I get that. My argument is: Apple would not exist today without the giants shoulders they stand on, Spotify (as an example) is a material reason why they're so successful, to whatever degree, and their insolence in admitting this isn't just morally wrong; its psychotically egotistical. This is, generally, the argument against all Ayn Randian philosophy; its not new ground, this is why Ayn Rand isn't really respected as an philosopher, and why her arguments also don't work in Apple's case.