If we lived in a world that was like the 90's computers era, where Apple was dominating the software front and by extension the hardware front. If people only ever developed for iOS, and most never bothered with even a cursory port to android; then maybe it would be reasonable to say that Apple's business practices make Librem or even Android unappealing to consumers and anti-competetive in the market.
But the evidence just doesn't seem to support that. Most developers are clearly willing to write two versions of their apps. Certainly the major vendors are. Apple doesn't sell their OS for any other hardware except their own, and there are multiple other hardware vendors, so it's not a case of "have to bundle an iOS license with every phone regardless of the OS shipped".
The conclusion has to be that there's something specific about Apple's combination of hardware and software that makes it compelling despite the lack of open access. So the question then becomes what is it that Apple is offering that open platforms can't or won't offer?