You might have had a point if SMS wasn't a first-class citizen on iOS and you can also easily use Hangouts, Facebook Messenger, WhatsApp, Signal, Telegram, LINE, WeChat, KakaoTalk and whatever else is out there

My whole point is that the existence of areas which don't interoperate doesn't harm the areas that do. iMessage is proprietary, but SMS isn't, and iPhones support both pretty seamlessly.

Remind me, does Bluetooth work with non-Apple devices again?

Of course. I regularly use it to play music in my car, for example.