The price of built-in text messaging whether SMS or iMessage (or a combination of the two) largely went to zero for most people in the US texting especially other people in the US by the time other apps gained traction. And services like appointment reminders and verification codes pretty much all use SMS in the US--whatever its potential security vulnerabilities.

So I use 2 or 3 other apps for mostly a few international people I know but basically everyone I know just defaults to SMS/iMessage.