RCS is a modern replacement for SMS/MMS with end-to-end encryption (E2EE) support via MLS. We have SMS/MMS so we need RCS to replace those. Similarly, carrier-based calls are there and should get replaced with a newer standard with E2EE.

RCS is what GMS Android and iOS provide so that's what people need to talk to non-GrapheneOS users.

Which non-carrier-based messaging app or protocol do you think we should include and why? What happens if we decide that's no longer the best one wand want to get rid of it? Apps included in the OS cannot be easily removed but rather only disabled by default for new installs and phased out for new devices. Otherwise, users would have their working setup break.

I agree. The OS maker has a ton of power with the default apps and putting SMS and phone calls as the default communication platform of the OS is contrary to the goals of giving people more privacy.

And the OS should come with a GrapheneOS Monero wallet to compete with Google Wallet/Apple Pay.