What is it replaced with?
The handwriting was on the wall when Google redesigned hangouts to make it look more teen friendly a few years ago, but unfortunately they removed some useful functionality and made it far worse on a smaller (laptop sized) screen.
It's too tied to phone numbers and mobile devices (have fun getting those RCS messages on your tablet or laptop or when you have no 4G).
It's also too tied to carriers, who might introduce weird billing "RCS messages to abroad cost $$$$". Its also without any end to end encryption, and seems to have an odd 1 or 2 second delay delivering messages which means truly interactive conversations aren't really possible.
Each of those things alone makes it less attractive to one usergroup. Together, I'd guess it'll never see widespread use.
Most people I know (US perspective of course) chat mostly on iMessage and SMS. They'd love RCS.
SMS has a delay of 5+ seconds in most of Europe, so isn't really good for rapid fire conversations. Many phones aren't correctly configured for MMS (picture messages) either, and some networks don't yet support emoji.
Nah, but most probably by Messenger or WhatsApp, Telegram maybe...