From what I understand with Apple’s fallback (or like Google’s Message app does), if RCS is sensed by the other non-iMessage user, then RCS will be used, if not right now it would still default back to text SMS but then lose some features like hi-res photos etc. Just don’t know how it will work for me where I am on iMessage on my iPad, but when out with my Android phone will the iMessage’s wait a week until I turn on my iPad again. Would be nice if there was a proper presence sensing, and it routes to there. That may be possible with RCS, but we won’t know how Apple plans to use it, and they are not going to want it to be as shiny and nice as sending an iMessage…