Discovering you're not limited to sms was nice, when now you have to have an aside - "hey, have you ever heard of signal?"
1) Spam (~40%)
2) Transactional messages (~40%)
3) Conversations with old (45+) relatives (~15%)
4) Conversations with people I barely know (parents of kids' friends, people responding to a web market listing, that kind of thing) (~5%)
Google Messages (the default SMS app on newer phones) uses RCS if available and overlays E2E encryption on top using the Signal protocol.
Most people will use the messaging platforms that they need to have installed to get through their days. It's nice, ofc, to have friends who are privacy enthusiasts - but Signal main goal has always been to go beyond that group.
but you are right it doesn't have much use for children