RCS only works when its connected to the internet. Not everyone has unlimited data, and the 128 kb throttled data after using up the fast data is slow as molasses that sites would fail to load and it's likely a reason why messaging would just revert back to SMS/MMS. (And some people don't even have the "unlimited 128 kb/s" after their fast data runs out)
They need to make RCS not require internet. Or carriers need to stop being greedy and just not count RCS data usage as actual data usage. (The data use is so insignificant it should not cause congestion anyways)