Your argument is very weird. Most of the time, if you are in location without cellular data, it is that you don't even have a good cellular coverage. In that case, especially, people are used to always use wifi and so use WhatsApp, signal, telegram, Facebook, instead of unreliable sms.

Also, if the goal is to have rich communication, like sending video and photo. If your network provider does not cover you for cellular data, I don't see how, the same content will go through anyway as it needs data...

The only difference of RCS compared to messaging service is that you use your phone number and the service is provided by your mobile telecom provider, but otherwise it is technically similar.

One last point is that, here, in Europe, people does not trust phone number as being a permanent reliable identification. Not so long ago people had to change numbers regularly when changing of provider or of offer.