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I don’t see how you’re not suggesting yet another competing platformI'm not suggesting another competing platform. I'm suggesting an already-widely-deployed platform (RCS) that is standardized, has been around since 2008 or so (older than iMessage!), and is the actual real elevation of SMS, not an Apple-proprietary protocol.
> If iMessage was cross platform
It's not, and that's the problem! Making it cross-platform would at least partially solve the problem, though it does mean Yet Another Messaging App I have to install.
> If you want secure cross platform messaging, use any of the other platforms built around that feature.
That's a huge pain in the ass. Not all of my friends have Signal, and I'm tired of evangelizing. Not all of my friends have WhatsApp, and I'm not interested in evangelizing a Facebook property. I don't use Telegram, and very few of my friends do, and I'm also wary of evangelizing a messenger that is not secure by default, and that has what is IMO a sketchy, secretive history around its cryptography.
Looking through my Messages history, there are 27 people I communicated with over unencrypted SMS/MMS over the past month. Only one of those people has Signal (not sure why they SMS'd me, but that just further illustrates the problem of how it's difficult to remember to contact people using the "correct" means), and maybe 6 or 8 of them have WhatsApp (for whatever reason, they/I don't really use WhatsApp often).
Why is this so hard? Apple! If Apple supported RCS (with the E2EE extension), literally every single one of those 27 conversations would have been encrypted, without me (or, critically, the other party, whom I have no control over) having to do anything special. As much as I love blaming Google for many things, they cannot solve this, because iMessage is a closed protocol that they are not permitted to implement, and the messenger app that is in the default install (the iMessage-supporting Messages app on iOS, and the RCS-supporting Messages app on Android) has a huge easy-adoption advantage that third-party messengers will never be able to match.
You can argue a bunch of "shoulds" here, but I'm talking about what the reality actually is.