While iMessage might be very popular in the US, here in my country I don’t really know anyone who uses it. It’s not that common in here since most people use Android, even Apple users don’t use it. Instead, most people here use apps like WhatsApp and Telegram.To one reporter who said he was having an issue texting his mother on an Android device, Cook said "Buy your mom an iPhone."
For me, most of the people I know use Android and I cannot get all of them to buy an iPhone just so they can use iMessage
And I do know some people who use iPhones, but they don’t really seem to care about iMessage/FaceTime.
That being said, I don’t really think adding RCS support would change anything. I think most people here will just continue using WhatsApp and Telegram still.