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?
The walled garden making it very hard to switch platforms, see comments here: https://news.ycombinator.com/item?id=38883393
Also, it’s almost impossible to create a freedom-respecting phone due to vendors refusing to release the hardware specs, https://news.ycombinator.com/item?id=26081818
> They don't forbid developers selling versions of their apps on other platforms.
How about web apps?
https://news.ycombinator.com/item?id=14864131
https://news.ycombinator.com/item?id=39388218&p=2
By the way web apps work great on a Librem 5.
This is the only item that answers “what does Apple provide that open platforms are unable or unwilling to provide”. So now my next question is if that’s what preventing open platforms from succeeding, why aren’t our governments working to force low level component suppliers to open up? Surely open specifications for the baseband modems and other necessary phone components would do a lot more for open platforms than the ability to install yet another chrome reskin browser on iOS.