None of my older relatives use email. They can text however (or iMessage, since it is transparent). Not everyone cares so little for friends and acquaintances.
You misunderstand. I have had to use various apps to communicate with people I dont need to talk to again, e.g. hotel and airbnb hosts in different countries which use these services. Like the time I had to install whatsapp in Spain to get into a boutique hotel. Sometimes you are meeting someone, e.g. a potential client who only uses signal so I install signal for our meeting and promptly uninstall it after we are done communicating. I have also had to install signal when coordinating with others for a hackathon and dumped it afterwards as we all had email addresses.