I can expect anyone who has a cell phone to be able to receive SMS. When I first started using a cell phone in 2007, that's all that was available. Or at least, it's all I knew that was available. Every phone I've owned since then has had the capability to use SMS, and I've never had a need to use anything else. So I don't see why I'd bother with something like Signal or WhatsApp, when there's no guarantee that the person I want to talk to will have it, and they don't offer anything I need that SMS/MMS doesn't already provide.