GrapheneOS does not want to adopt RCS for public perception. It is still a better option than SMS/MMS and providing it is beneficial. GrapheneOS will still recommend using better platforms and protocols but RCS being bundled and cross platform are great reasons to implement it.
Suppose GrapheneOS becomes highly popular. Then I have to fight the average Joe who insists RCS is fine enough.
I understood from other comments that RCS is the default communication channel in N.America an I can sort of see why you want to give people a somewhat less worse option here than Google's Messenger. But RCS should not be the default imho, because lazy people gonna be lazy. So I hereby endorse you to gently push newcomers towards a safe and privacy friendly default by pre-installing Signal or whatever meets the bar and give it a prominent place.
Whether or not you tolerate using RCS is up to you. Not supporting it for the sake of having a worse default option on the off-chance you can use the to convince them to use a better option is not a good reason to not support RCS.
Either you will have people content with SMS, or you will have people content with RCS. Which is better is obvious.
I am not on the GrapheneOS team but the community does push better options such as Molly/Signal and RCS is usually only advised as a fallback messenger.