I don't think it's too late as iOS finally supports RCS. But so far Google hasn't shown willingness to let unsanctioned clients connect to Jibe.
In practice, RCS is run by carriers in most of the world. They connect to hubs, the same way SMS hubs work and also have Universal Profiles.
Jibe is not small, but it hardly runs the worlds RCS. Maybe you're conflating the US with the world?
The US was actually the only market where a federated RCS was tried at scale for a few years (the CCMI) but all carriers eventually gave in as the UX was poor and unreliable.
To my knowledge there are only two other non-Jibe RCS "islands": China (that runs RCS solutions from national providers like ZTE) and +Message in Japan. +Message is on its way out, as carriers are now pushing subscribers to Google Messages and Jibe, anticipating the iOS support.
And Apple is in on it: MNOs don't have a choice here, they now need formal agreements with Google (Jibe is paid through RBM revenue share) and IMS configuration and (de)provisioning workflows that are sanctioned by Apple and de facto tested only against Jibe.
Apple's communication around E2EE in UP 3.0 is also directly following Google's work on replacing their ad-hoc Signal implementation by MLS.
That’s what makes Telegram and WhatsApp great
XMPP installed on the handset by default should be fine.
