I think it's not quite as bad as you're making it out to be. If Apple's abuse of their position became truly intolerable GP is correct: people would stop buying their products. Apple knows this, and that limits how aggressive they can be with their anti-consumer nonsense and forces them to keep innovating.

So market forces do still work on Apple, just not as efficiently as they otherwise would in the absence of these artificial barriers to competition.

Software is somewhat unique in its ability to act against the interests of the person who owns it. Trying to design a fridge to not refrigerate your competitor's food would be impractical and probably easy for consumers to bypass. But designing an operating system which won't run your competitor's software? Trivial, and very hard to bypass. Because of this, companies that write software include anti-consumer features like that all the time to the point where it's almost expected now.