It will be interesting to see how long it takes for RCS to be a true SMS replacement though. I suspect a lot of carriers will drag their feet. Also, it seems some may charge extra for RCS over SMS, so this will be a factor to consider.
The key word in my post was "universal", not "cross platform". Most cell phone users do NOT have Messenger on their phones and never will. Both WhatsApp and WeChat which I referenced in my post are far more popular than Messenger, but those are far from universal too.
In contrast, once iOS gets it, RCS will be universal across iPhones and Android phones with no third party app required.