Your take on the consumer perspective ignores network effects. Lots of people, particularly children, buy iPhones just to avoid the dreaded green bubbles. Adults may be better at handling this, but it's hard to tell children to just use Android if that means they'll be bullied. I don't think it was originally Apple's intention to conscript legions of schoolyard bullies to increase their market share, but it's certainly happening, and they certainly don't mind that it is. Interoperability (particular opening up iMessage and iCloud to other devices, the same way anyone can use RCS or Google Photos from an iPhone) would mitigate that.

Your take on the developer perspective is a bit confusing. Everyone knows it's a closed system, but that doesn't mean it's not a monopoly. In many ways, it is a monopoly.