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.