The intended purpose of RCS was to be federated between carriers, just like SMS. If you centralise it then it's just a worse version of any of Google's abandoned chat apps. But there's a realpolitik reason to insist on centralised RCS: because it's already got the reputation of decentralisation, it makes it look like Google is better than it actually is.

Yes but there are different specs that “compatible” apps will have to adhere to, basically until everyone is always running auto updating software to the latest version supporting RCS 4.0

Long story short somebody preferred to “ship it” instead of waiting to figure out how to make encryption compatible between iOS and Android. I guess there’s some differences in how the key rings work, but Signal can figure it out and /they’re/ open source so it sucks they ever supported unencrypted messages. They just wanted to say it technically worked for rich text messages and reactions.

It has it in its current documentation (maybe 3.0 too? I forget, but it's new this year), and Apple and Google have had messages to themselves encrypted for a while (but not to each other, which is what the RCS docs are intended to resolve). It took many years to get it documented though (despite loudly claiming RCS is more secure the whole time), and AFAIK none yet implement it. Definitely none with all the documented things it needs to show to be compliant with the spec I read, e.g. key verification - I certainly don't see any of that in Google's "Messages" app! Though hopefully that will all change.

But honestly, in a non-open ecosystem, can you really trust that the near-exclusive two major players are actually playing by the rules? Apple has been relatively protective of its users on privacy stuff, but I've lost all trust in Google at this point.

Almost none of my RCS conversations are encrypted. Who cares what's in the spec if nobody uses it?