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people using the core SMS function on iPhones are automatically opted into iMessageThis isn't the case. you have to make an iCloud account and Messages settings has to be set to use that iCloud account.
Yes, you can do that by setting up a new phone with your iCloud account.
But -- surprisingly -- you do not have to.
You can set up and use most things about an iPhone without iCloud.
You'll be missing FaceTime, iMessage, iCloud Backup, FindMy, and a few other things, which are parts of "services" rather than parts of the phone.
// This is fresh in mind, as I recently bought a slew of "real SMS" phones for the workplace, since robo-fearing 2FA providers mostly refuse to work with e-SMS (and god bless vendors who won't use TOTP but think SMS is dandy).
> then get a worse experience when communicating with groups containing people who don’t have iPhones
And people who opted into email have a worse experience exchanging notes with families that don't have electronic devices.
In general that's how lowest common denominators work.