There is no need for carrier based messaging anyway. It’s like with fax machines, sure there can be new Internet based protocol for fax machines that phone/fax carriers would start to support and new printers being forced to adopt, but if it’s Internet based why not just use email.
At least SMS is using voice PSTN network and is purely device-to-device, so it can stay for emergencies. But RCS is purelt Internet based messenging that was abandoned because of no benefit over what we already have like XMPP, until Google started forcing it. Right now Google Messages are the only RCS app on the market and the only one available to support it, because carriers just delegate servers to Google for hosting.