The average person just has no idea about RCS or protocols in general and are incidental adopters of it just like SMS. Sometimes these nerd debates about platforms and protocols emphasize technology features over actually connecting with people or doing something productive on said technology.
I'm a nerd. I know vaguely what RCS is because I had a discussion in 2019 with a friend about it. Do I have it? Do I use it? I have no idea. Is it an app or just a protocol that happens behind the scenes? I would assume the latter. My phone's a few years old, isn't everyone's? Probably that means I don't have it. No way to tell and I'm not going to bother trying to find out.
So.... proprietary data collecting thing owned by Google, service that requires phone number to sign up, or service that does not even pretend to be E2EE and (worse) routes chat traffic through multiple potentially-adversary-controlled servers on its way to you?
Reality check: nobody cares about you if you do not comply with societal "common denominator" norms.
As unfortunate and bullshit as that sounds, it will always hold true. This is the reason I have Whatsapp and Discord work profile sandboxed and highly restricted running through custom HOSTS ruleset + NetGuard. I concluded these are the 2 platforms needed to minmax information "freedom" and social compliance.
Most of my friends use Signal. Honestly hadn't heard of RCS till now. Either my phone only supports SMS or I'm too technologically incompetent to enable RCS.