So many of these comments are pulling up the other encrypted alternatives that you can use between iPhone and other platforms. But few seem to actually be addressing the problem of actually getting other non-tech savvy people to use this stuff because they don't actually see a problem with what they have.
You may not realize it, but not everyone is thinking about whether or not their messages are encrypted. My own family looks at me like "🤨" when I try to convince them to use something encrypted, like I'm trying to hide a crime or something. And I've only gotten my parents to use other services (WhatsApp and Facebook Messenger with end to encryption turned on) by digging my heels to get them to stop using SMS. I still haven't convinced my almost 16-year-old sister (she doesn't really message me that much anyway. But she's in that phase where she thinks she's all independent, and her first places are the simple stuff she knows).
Might I add that digging your heels at every attempt for someone to use SMS isn't socially acceptable. I've only done it because they're family and I love them