Comment by B1FIDO

SMS is very widespread in the United States.

All the B2C services I work with are sending SMS to my phone. Not RCS, not iMessage: they are sending SMS messages.

All the MFA providers, such as Twilio and Okta, are sending SMS.

All the political campaign spammers are sending SMS.

All the reminders for appointments and bills are sending SMS.

All the notifications for apps where Push isn't good enough: they're sending SMS.

If user-to-user communication is using iMessage then that is fine. I have noticed that only about 2 of my human contacts use RCS, and at least 2 of them are using iPhones and not Androids for it. So that's some anecdata for ya!