So those of us who happen to own iPhones, and have friend who have Android devices, and would like to have (e.g.) group chats can go pound sand? Or try to get all of our friends/family to install yet another app? What is the main argument(s?) against implementing RCS? That it doesn't have E2EE? Neither does SMS, so RCS is no worse in that regard, but seems to have some extra nice things.
The reason is that SMS is dying and being replaced by better encrypted clients. There is no point moving to RCS which just prolongs this death and makes the current situation worse since Google has their own proprietary version of RCS.
RCS Only has advantages over regular SMS.