The comments here are so bizarre to me. I think this must be a USA thing. I had no idea signal even supported SMS, nor do I know anybody who uses SMS
Where do you live where people do not use SMS? In most countries I've been to and lived in in Europe SMS was very much used.
SMS is typically used for notifications-style messaging (or spam), so the impact of this change is probably minimal for a lot of people there. Even chat-bot/support style messaging in the B2C space are moving to WhatsApp (or equivalent).