Signal, WhatsApp, etc. don't come close to what iMessage offers seamlessly. For iPhone/macOS/iPadOS users, SMS is garbage. Yes, it's a closed system; but it works without a cell signal or roaming charges anywhere in the world over WiFi for free.
RCS seems like a half-baked attempt at catching up with features that've existed since 2011 on one platform. Even third-party messaging apps fail miserably compared to iMessage.
In what way? I know WhatsApp compress media files, which is why people use Telegram when high quality matters. Is there anything else? In any case Signal and WhatsApp have UX barely better than SMS, the real comparison should be against something with legitimately better UX. Is iMessage better than Telegram?
It's not useless, but vastly better for short text conversations than anything else. Not surprising, it's SMS.
If I were Apple, I would not budge.
I haven't used iMessage, so I don't know how it behaves, but my own personal RCS experience has not been good. I am on Google Fi and make heavy use of Google Messages and specifically their web app. I have a Pixel 4a 5G - a good mid range phone. Three Messages configurations are available: regular SMS/MMS, RCS (i.e. Enable Chat Features), or SMS/MMS routed through Fi; RCS is not available if you route through Fi!
I have a couple of really long message histories, and with RCS enabled, it got to the point that I could not keep the web app connected with the phone, open chats, or in some cases see/respond to messages; I'd try to send a message and it would sit at 'sending' for 5 minutes or more, making it very difficult to have an actual conversation with someone. Turn RCS off, and the send/receive delay disappears. Route through Fi as well, and the web app works a lot better and it doesn't get stuck opening conversations.
As another commenter pointed out, it's probably better to use an actual chat application instead of Messages for this. But that's not what I use :shrug:. And my experience, on my phone, is that RCS is not usable at all, and I'm just on SMS/MMS as a result.