They don't falsely imagine that they have the ability to modify the device 'as they see fit' in the first place. To do that, you'd have to do more than just 'hire a programmer'. If you really have the resources to do that, then nobody, not even Apple, is stopping you from hiring a team.
That's the crux of the matter though. Apple can stop me from hiring a team. They can do anything they like, because they have retained ultimate control over my [1] device, even after purporting to sell it to me.
[1] - "My device" is only rhetorical of course - I don't actually own any iOS devices for reasons that should be obvious by now.
But really, if you have the resources to hire a team to build a Mobile OS, why wouldn't you just commission hardware from someone else, rather than running it on iPhones?