I mean it's similar with Signal: you can send Signal messages until someone in your contacts (which are most) doesn't use Signal so you have to send an SMS.

The problem is that Signal prefers to also set itself up as the default SMS app and doesn't clearly show which messages in the feed are what kind. As someone who has to explain this difference to his father about twice a month I don't like this "feature", it's utterly confusing to people who don't know or don't care about the distinction between web-based messaging, SMS and MMS(now defunct here) when they want to send a picture.