In the end the reason apple will never adopt RCS (at least in its current form) is for this to work Appel would need to expose an api that lets Google inspect if a given phone number is an iPhone and check if that phone is online.
Furthermore while googles (out of spec) extension does end to end encryption it only encrypts message contents google still knows who is messaging who, when and even how long it takes them to send a message back. It also does not encrypted group chats at all.
You need to ask why is google willing to pay the (large) costs of running RCS? well the reason is this data about who has what phone, when are they online, what the exact ping time from phone to googles closest servers is all very $$$ for targeting ads.
If apple adopt RCS as it is today it would be a massive breach of the current privacy terms. The only way they could adopt RCS would be on a per contact opt in system but RCS is not written in such a way that when google queries then to check if a phone is online (required for RCS) that apple can validate that this query is coming from a contact the user has approved to talk to over RCS.
In the end RCS what filled with loopholes to expose data since both google (and the service providers) all what this data as it is $$$$. And running a message service costs $$ so non of them are willing to run the servers unless they can make a profit and the only way to make a profit form RCS is to use that data.
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Google and Apple could work on an alternative privacy preserving protocol like they did for contact tracing. Modern crypto would allow for such systems to be built but there would be no money in it for google so why would they spend $$.. we all know what happens to google services that don’t make money (see… endless list of dead products here…)
and the same is true for apple why would they host a messaging service for non-paying customers. Right now iMessage makes economic sense to run the servers but it would not make any sense for them to host messages that are between android users.