SMS is as convenient as it is universal, allowing you to text anyone, regardless of their device or messaging app. But SMS messages are an open book, easily read by your mobile service provider, your gov't, and hackers. Here is why we don't recommend SMS: https://proton.me/blog/stop-using-sms
Why you should stop using SMS | Proton

Despite its popularity, SMS is a dangerously insecure way to send text messages. We examine why and discuss what you can do about it.

Proton

@protonmail Hear hear.

The problem I (and I suspect many others) have is trying to get everyone I know on board with any one E2EE platform.

And given the basic security practice of not having apps on your phone that you don't need, I'm not about to get a handful of texting apps just to communicate to various friends opting to use one proprietary method over another.