This whole situation sounds like a conversation between two horse cart owners stuck in the street arguing over whether grass or hay (or maybe cornflower petals?) is better for their horses, but all the whole time completely ignoring that an entire fast and advanced metro system has been built across their city during the past few years, with easy interchange between different lines.
SMS was the result, over 30 years ago now, of a happy coincidence between there being a few bytes spare in the GSM signalling protocols, and the realisation that a messaging system would be a handy thing to have and could also happily fit into that bandwidth.
But that was a long time time ago now, and mobile networks, and the internet, and expected uses, have changed massively since then. You look at RCS and you see that it has taken years and years and a crazy number of multiple iterations to even start to begin to get off the ground, and that, for no good reason nowadays, it needs individual mobile network operators to be involved and for them to provide equipment to support it (and they would probably even have seen it as a cash cow to try to coin in from, as they did with excessive SMS message costs at one point, if they could get away with it).
This is a ridiculous situation nowadays, where internet access on mobile phones is now entirely the norm (at a range of speeds and capabilities from GPRS upwards, but essentially becoming 4G+ from now on), and the mobile networks are now essentially mostly just data pipes, with fallback to the venerable older technologies for SMS and (sometimes) voice.
Any updated messaging protocol nowadays should just be a normal "over the top" internet app, the same as everything else, and of course E2EE by default. Messing around with ridiculously complicated gubbins way down at the mobile network operator level (why on earth?) is like trying to paint the cart wheels fluorescent yellow in the hope that this will make the horse cart go faster. (And being harnessed to an untrustworthy data-reaping 'partner' (Google) to gain what should be essential basic features such as encryption, with a lack of clarity of what happens at the boundaries between their black boxes and the rest of the network, is definitely a no-no situation.)
We have privacy-focused apps like Signal and Threema nowadays, we shouldn't be trying to prop up a now outdated and overly-complicated system like RCS (or apps with less well-regarded technology like Telegram, or those in the clutches of other data reapers like WhatsApp). If anything, the focus and effort should be on trying to make Signal's well-regarded technology into a proper interoperable and open standard that all communications apps can use, in an interoperable way.
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