Wow, I've had RCS for a while and thought everyone had it now
Most providers in most countries support it now, though it took a couple of years to roll out. The question is what fraction of people use it?
I've never enabled it (or "registered" as the settings call it). That's partly because literally nobody has ever mentioned it to me, or mentioned it at all within my earshot. Partly because by the time it first appeared I, like pretty much everyone I know, was already using other apps for multimedia chats which worked across platforms and across networks, so adding another system to the 3 or 4 I already had, that many people could not even in principle access, wasn't really a priority. And partly because with only a couple of rather uncustomisable apps supporting it I would have to give something up to use it, when the benefits were unclear.
But to be honest, the case for using it didn't seem strong to me. If I want to just send someone a text, SMS is fine (unless they have a non-UK number, when it will be expensive, especially since regular SMS have been free in the UK for so long that it just feels wrong to pay for one
). If I want to set up a group chat I'm going to use a cross-platform system, because almost certainly the group will include both Android and iPhone users so there's no choice (MMS is universal, but in the UK MMS cost £0.30 - £0.65 per message sent, so nobody uses them for group chats). So the only real use case is if I want to send a picture to one person, but since RCS isn't available on iPhones, or with most 3rd party message apps, or if you haven't taken the extra step of registering with the service, there's a very good chance that the person I want to send it to can't receive it. Hence I'm actually going to check whether they are on WhatsApp first, because it's more likely to succeed, and if they are I'm not then going to check whether they can receive RCS (and I definitely don't want to use an RCS capable app if it will fall back to MMS if RCS isn't available - I don't know whether they do, but I can imagine an app developer thinking that was being helpful...). So put all of that together and it's never really seemed worth the effort of registering and using a (to my mind) otherwise lesser message app on the off chance that RCS was ever useful.