It's easy to say that "you should do x" from the bleachers but when you're in the arena you run up against reality. For example, Signal had a blog a while ago about how they tried to avoid the sms features, actually for privacy reasons, but they found people just didn't use other alternatives. Here's a reddit thread of users advocating for SMS support
https://www.reddit.com/r/signal/comments/y3ymfl/keep_sms_sup... .
So it was the best of all the available options practically, if they wanted to grow and retain the users.