https://xkcd.com/927/
Relevant xkcd.
Whatsapp, Telegram, FB, iMessages are NOT standards. They're closed systems controlled by a single company.
XMPP is a standard from the days of ICQ, AIM, MSN, etc. Unfortunately it failed.
SMS is a standard for mobile phone text communications. It's too limited now.
RCS is the evolution of SMS.
I don't think RCS is a redundant standard, it's a necessary evolution of SMS. It's a standard for a decentralized system. I know Americans hate their carriers to the point they'd rather have everyone's data in a single SV company, but the solution is to fix your carriers. Not to give all your data to a company (Facebook) that had been proved to be less than trustable about it.
That's what the term "Defacto Standard" is for.
RCS is still focussed on phones only, so it’s a non starter IMO. But it’s driven by the same companies that charge the equivalent of $1000+/mb for sms delivery, so it’s unsurprising that customer needs are not really their first priority.