The whole iMessage/RCS conversation is really only relevant in the US; in other countries basically everyone uses WhatsApp or Kakao or LINE or whatever the local favorite is. In the US, there is no industry-standard RCS. It's theoretically a carrier-based messaging service but all of the carriers outsourced it to Google so, as an alternative to iMessage, the option is a proprietary extension of RCS running on Google servers, something that is exactly as open as iMessage itself.
If you want a true industry standard way to send messages to people, the iPhone has had that since 2007: email.