tango I think someone you'd have a conversation with would probably care about privacy to some extent, but it also probably depends on what that person means by privacy, which is why threat modeling is important and the need to distinguish between what could happen and the probability of it happening, a phone operator is able to access all your data but the risk of them posting your private data online to harm your reputation is weak. Unencrypted messaging pass unreliably over mobile network is enough for me to avoid SMS and because SMS phising is way to easy.
Without an effective and widespread means of communication, it's pretty hard for developers to get the word out about their products, WhatsApp got a lot of publicity because Meta promoted it on Facebook, which has billions of users, and more people heard about Signal when Elon Musk talked about it on Twitter, not everyone likes to look for novelties on their own.