I didn't know that green-message ostracizing was a thing until this thread.
The advantage of iMessage is that it's dependent on data, not voice. So if you are in an area with no cell service, but you have wi-fi (like the motel where my brother and I stayed in Mexican Hat, Utah, last summer) you can still communicate. You can also message with an iPad, iPod Touch, or Mac laptop if you have wi-fi or a wired Internet connection. It used the device's logged-in AppleID, not a phone number, for the communication -- for the most part, my mother- and sister-in-law use the same AppleID on their phones as my sis-in-law holds the account and set up her mom's phone, and a couple of years ago they had issues with messaging coming from or too the wrong person.
Not sure how Google/Android would leverage a messaging system that uses Apple's protocols.
While it is true that you can turn off read receipts in iMessage, you can't for Facebook Messenger.
I know nothing about WhatsApp, but wouldn't it be a cross-platform data-dependent messaging system?
There's always Google Voice, which will run on any device as well as through a web interface.