No one gave a crap about RCS and no one was supporting it until Google decided that they needed a new chat app because they hadn't made everyone switch in a while.
RCS isn't a Google only thing. And it isn't an "app". It is disappointing that people don't understand that RCS is a great replacement for SMS/MMS.
MMS did, which far predates RCS.
Everything about the concept of a phone number is confusing to me. It's a string of digits that if someone guesses, they can activate the most active notification your phone has (ringing), at any time, no matter if you know them or not. Better yet, depending on your notification and MMS app settings, they might be able to make a dick pic appear on your lock screen on a whim - big spammers of this seem to get marked by the carriers and apps pretty quickly, but for a more targeted one off, still easy.
As opposed to tcp/IP based chat apps that basically require a bilateral human-initiated handshake before someone can message you...
Tying and account to a phone number is a privacy nightmare.
I guess Facebook/Meta does it for easier social graph extraction/profiling, while Signal tried to hand of verification to precent spam. But for the sake of this argument, we may just assume all of them are evil.
And there is one singular app which supports RCS.
In many ways, it's a regression from SMS. In that SMS is somewhat universal, and RCS is so specialized it's almost worthless.
To put it another way, Google can't kill SMS short of literally removing the app from Android because it's not their infrastructure, but if they shut off their RCS servers tomorrow, it would be dead for good. That's a Google-only service.
They could just layer their own chat platform on top of Google Messages but we all saw how Google's IM business went along: Chat, Hangouts, Alo, Meet etc. So they muddied the water so deep (to a carrier level) to make it look like it's Apple's issue for not adopting RCS. And people actually fall for it.
Nobody wanted RCS. Even carriers don't want to maintain RCS. They just use Jibe. And that's exactly what Google wanted. My RCS communication with friends don't even show up in carrier's usage. How is that ever different from iMessage...
You know who chose to selfhost their own RCS server? Yes, Chinese carriers! They call it 5G Message. New ad delivery channel for businesses hooray! Instead of plain text and a link, now your campaigns can even have MENUs inside! I can send SMS to a Chinese number, I can send iMessage to a Chinese number, but I can't send RCS. Truly "Universal" profile.
It's a complete shitshow.
Have you tried to restore a backup? You cant, unless it is uploaded to google cloud. No google account, no backup. (including the adress book that's tied to the account, since you are asking, they changed restore rules recently)
Have you tried denying adress book access? Whatsapp barrs you from starting a conversation. But there is the workaround with https://wa.me/+phone ...except for WhatsApp web
Have you ever tried putting Whatsapp in an Android work profile? Now try to export a chat!
Every once in a while a get the task to save all pictures of a conversation and it is usually a pain (If you think its easy, try again in Androids work profile).
From a UX perspective I would never mourn WhatsApp