They don't want features like end to end encryption because it relegates them to being a dumb pipe and limits their ability to innovate e.g. parsing SMS messages for advertising purposes. Also many carriers are closely tied to governments who want to be able to monitor communication.
I've worked for a number of telcos over the years and can assure you that they don't have the interest of users at heart.
As such, RCS isn’t the answer simply because the carriers were involved. SMS’ successor should basically be a vendor agnostic WhatsApp or Signal complete with E2E built into the base spec instead of duct taped on as an extension that can conveniently be ignored.
Great. Build something, then call me when you have the default messaging app on board on every platform save one.
From my perspective, RCS has the best shot at this and the perfect shouldn't be the enemy of the good.
Except stats don't back up what you are saying, at least not in the US. RCS is no less trash than SMS, it's actually an improvement over SMS. They could come together, along with carriers, to create an even better standard but one party is holding out...and blocking any innovation (hint, it's Apple.) I don't get how people are ok with them hijacking an open standard.