There are countless ways of making it happen by installing apps. And the user experience and integration has improved. But I mean out-of-the-box, universal functionality. So your 94 year old great grandparent can use it without anything.

It is, and it can work; but it also doesn't typically get the fully blessed experience a first party app does.

Duo integrates into the latest Android like FaceTime integrates into iOS, it would be nice if Apple made FaceTime / iMessage available on Android (lol, never gonna happen) or allowed Duo to integrate into iOS like 3rd party apps can in Android (like WhatsApp video calling does). Probably the biggest feature I miss from Android since moving to iOS.

Apple is probably doing more harm than good at this point for themselves by keeping iMessage/FaceTime so locked down to their hardware since their revenue stream is trending more towards services and less weighted in hardware.

I'm not sure if Apple ever explained why they didn't open up FaceTime? I seem to remember that when they presented it they said it would become an open standard?

There have been persistent rumors of an issue with patents.