SMS is a defacto carrier tech that people expect to work, people also don't expect e2ee or any kind of privacy scheme on SMS but it might be a place to hand communication off to a different tech. Whereas RCS leaks meta data to the goog or worse (I don't know the ins and outs of RCS) but by default I would trust it less then signal, who may also collect meta data leaked through the goog, so I guess the real solution is to be boring and assume there is no security or privacy for anyone of reasonable means.

The world unfortunately uses SMS for 2FA.