hahaha I would argue RCS is even less secure than SMS/MMS, because it provides a false sense of security. There is and never has been any expectation that SMS/MMS are secure beyond basic GSM/LTE/5G physical layer security (not end to end encrypted). RCS offer the false promise of end to end encryption, except it generally messages aren't end to end encrypted. The carriers can still peek inside the messages, assuming encryption can actually be negotiated between the send and receiver. This is ultimately just Google trying to change the rules to make It's piss poor messaging story not so piss poor. Google has proven time and time again they can't do messaging services.
As for RCS leading to no iPhones. Sounds like a you're having a wet dream or a stroke, it's hard to tell. For some of us the only reason to not get an iPhone would be if Apple stops selling iPhones. It's a dick move but I honestly don't care if I can "message" people with Android phones. I carry and use technology for my convince not the rest of the world's. If you're Android device can't communicate with my non-Android device, that's a you problem. If I need to communicate with you I'll call, schedule a meeting, or use a cross platform service.
If Google and/or the phone industry introduced a messaging spec that 1. provided true end to end encryption, with no way to disable it, 2. wasn't a massive marketing channel and 3. wasn't tied to a 140 year old technology (phone number). Hum... I don't made something like iMessage, Signal, WhatsApp, etc. etc. Then we can discuss moving all person to person messaging to it.