Good point… I found that apart from technical interoperability it often works pretty well if you explain to your friends that your alternative (signal,Matrix, whatever…) is just that. An alternative which is always good to have just in case. Don’t try to force them to uninstall WhatsApp, even if this would obviously be the best choice. Instead encourage them to try the alternative and keep WhatsApp in case they don’t like it. Test it with them. In practice this often means they find out that it works just as well and does not hurt to have on the device. Even if they don’t use it actively yet, the next time someone asks them if they have Signal (or whatever) they will be happy to say that they already have it. Patience is key.