Not sure, but does sending an RCS message to a user who's carrier doesn't support it or has locked it out automatically fall-Back to SMS? it seems like a mixed-bag depending on carrier and device (there appears to be device specific options to fall-back to SMS, but then I see users complaining that this isn't reliable if the problem is on the receiver's end.) - so lack of reliability (an guaranteed encryption) might deter Apple.RCS is practically a walking corpse. That said, apple should still adopt it into the default messages app. It is a significant upgrade over SMS and while the carrier lockout issues are dumb, people can always continue to use other apps like they already are doing if they are a dealbreaker. SMS is just as unencrypted as RCS is, it's not like it uniquely erodes your privacy.
They should probably just choose a new color for RCS to avoid it being confusing over if you're doing SMS or RCS, like orange or purple.
Apple is in the business of tech that "Just Works", implementing a protocol that sometimes encrypts other times not, maybe gets delivered, maybe doesn't - just would tarnish their product.
I mean AT&T is/was locked as to who you could message with RCS