I think the main point of replacing sms with rcs is that it would be the default preinstalled carrier-agnostic-protocol messaging app on everyone's phones. If signal could be preinstalled and be an open platform instead of using centralized servers then I think that would achieve the same thing. But because it isn't, it can't be the lowest common denominator that everyone falls back to if you and someone else don't have the same messaging app preferences.
Right now, you can say "use signal" all you want, but if you make a friend who isn't interested in installing a messaging app, you have to fall back to sms. Rcs is just about making a better fallback option that people won't refuse to download because it's ubiquitous and supported by even the default preinstalled messaging apps.
I hear what your saying, except RCS is not ubiquitous, is it? As I understand it, RCS defaults back to regular SMS if one party in the conversation doesn't have it, but IMO that seems to work against the benefits of RCS and delays adoption. I'm not saying Signal is the answer here and RCS has potential, but it seems like messaging is fractured and to ensure you're getting RCS like features, a choice does need to be made in terms of which app to use.
Yes, but I'm pretty sure that Google users their own proprietary model and their own servers. So they took RCS and built on it. Not to mention that Google Messages is a Google product...till they kill it or rebrand it, like they did with Google Allo. From Google's FAQ page on RCS (everything mentions "Google's backend"):
How RCS chats work?
When you use RCS chats by Google, messages are sent and received through Google’s RCS backend over the internet. Messages can either be delivered to or received from users on other RCS service providers. If RCS chats are provided by Google, but your recipient’s RCS service is with another provider, your messages are routed through Google’s RCS backend and then routed to your recipient's RCS backend.
Note, this doesn't work with any other (hypothetical) RCS clients, this is not a part of the RCS standard, it's a feature entirely proprietary to Google Messages.