I don't think it's that easy.
Yes, SMS is indeed insecure, and the argument, that a lack in security also poses a risk to privacy is perfectly valid.
However, the argument that high security results in high or reasonable privacy is - in my opinion - not valid.
Were that the case, clearly Facebook, Tiktok and GMail, all of whom use strong transport layer encryption, would be more privacy-friendly alternatives to SMS, which I think most would agree is questionable.
The higher level of security which RCS unequivocally provides, doesn't automatically make it a sensible option for privacy.
As of today, there is no open or auditable implementation of RCS. In fact there are very few implementations at all, with the vast majority controlled by Google.
Google does not only provide the bulk of client installations, they also basically run the server infrastructure. Most carriers don't actually run their own RCS services, but delegate this to Google.
How Apple implements this is unknown (to me).
Even if E2EE is used (which is neither guaranteed to be the case, nor easily verifiable), there is still a lot of metadata to be collected.
And Google is very much known the leading trader, collector and monetizer of metadata, which is a big problem for privacy.
Vice versa, even if SMS is in principle barely encrypted and very vulnerable, the infrastructure is run by carriers, many of whom are in legislations with much higher privacy standards than Googles, to whose terms you have to agree to practically use RCS.
In my opionion, this will only change once users are able to chose an alternative implementation and have some control over which infrastructure-provider they use.
Right now, this is not the case.
As of today, Google RCS is clearly the more secure option, but not clearly the more privacy-friendly option.
If you require privacy and security, you clearly should use another solution, there's a whole bunch of better options.