But since all the networks since 4G, there is no more low-level network support for things like SMS. Everything, including voice and messages, is IP- and packet-based. So the only thing the carrier does anymore is to authenticate that IP connection through your SIM card and bind your identity to the phone number. It actually doesn't really matter if messages are "network native" or through a third-party app, there is no more guaranteed timeslot and reliable delivery that SMS used to have.
And nowadays, RCS is also outsourced to Google by basically every carrier.
So RCS is the same as WhatsApp et al., only that the app you are using doesn't tell you that Google will monitor all your communications in addition to the monitoring your carrier does...