I suspect that carriers have been uninterested in RCS because they have always enjoyed the charging model for SMS/MMS - there are still plenty of people who don’t have bundled ‘limitless’ texts.
Mobile messaging has moved on though. Sadly into propriety services, be that iMessage, BBM (RIP) or WhatsApp.
WhatsApp is a great service and being platform agnostic has enabled them to eat market share - I just wish it wasn’t owned by that pool of rancidity we know as Facebook.
Switching to Telegram or Viber just isn’t a viable option because there is little point being in a chat on your own.
RCS is encrypted in transport but does not feature end-to-end encryption. Google says it will delete your messages on delivery - but it’s federated so you cant really guarantee that your messages won’t touch your carrier’s own implementation (when/if they get round to it).
It would be pretty trivial for a future government to mandate that UK-based carriers store massages that are sent via their servers for a period of a couple of years. They already do this in many areas.
So, I’m all for RCS - I just don’t see it being the WhatsApp killer.