particularly if you want to support all of the message-body features, of which there are MANY. it's very business-comms oriented.

I broadly expect third-party RCS apps to drop that though, like how essentially none support all of MMS. did you know MMS supports slideshows (I built support for this once)? 3d objects (mimetype model/gltf+json)? "timed text" (mimetype text/mp4)?