RCS wouldn't replace iMessage, it would be a new layer of fallback between iMessage and the current MMS/SMS fallback, so the security situation wouldn't change.
RCS uses your data plan[0], so it's unlikely carriers will bother charging per message. I already have unlimited international SMS and MMS on my plan anyway. Charging per message on RCS would just mean the protocol would fail and push people to the multitude of other options.
It would also probably regress iOS Apple’s customers have already paid money for.
How is any of that morally “the right thing”?