Email, docs, music, maps etc have reasonable alternatives and you don’t depend on others to switch (a gmail user will receive my email from tuta). But with WhatsApp either you conceal and use it or you just don’t talk to people (tried convincing them to switch to signal and it simply doesn’t happen)
How do you deal with it?
I’m the developer of a small messenger, so take this with that in mind. The honest answer from the other side of the fence: there is no trick. A messenger only works when the other person has it, and most people won’t install a second app for one contact.
What has worked for me, a little: not asking people to “switch”, but to keep one conversation somewhere else. One person, one thread (a partner, a sibling, a friend you talk to daily). If that one thread is pleasant, the second one follows on its own. Asking a whole family group to move at once fails almost every time.
The other thing that helps is removing the sign-up cost entirely. My app (FlickerTalk, Android, AGPL) has no account and no phone number: you open it, show a QR code, done. Chat history stays on the phone, and if the other phone is off a mailbox holds the message end-to-end encrypted until it’s picked up; we can’t read it. Limits, since this is a privacy community: it’s 1:1 and small circles only, no large groups; on a direct connection the other phone can see your public IP; and Google delivers the wake-up notification (not the content). Not a WhatsApp replacement for a 60-person family group. For the one person you actually want to talk to privately, it’s enough.