Well put, and I agree, I believe this is the one major advantage that makes iMessage truly unique. All of iMessage’s other major features can probably be found in one chat app or another. But the other possible unique thing about iMessage is that it may be the only app to combine all the important features that people have described in this thread.
Maybe someone has mentioned this as I didn’t read the whole thread, but as far as being built into the stock texting app, apparently, Google is finally starting to catch up to the idea with their Google Messages app, which can now combine regular sms texts with data texts. But it’s not the default texting app yet for all phones and/or the function has to be enabled, or something along those lines. I imagine this will eventually change though and it will become default like iMessage. It’s also working on end to end encryption supposedly.
But as of now, I believe iMessage is the only app that has the entire feature set, plus is built right in to the default texting app, plus is from a company with a business model conducive to protecting customers’ data and privacy.