My wife won't use Signal because it includes a crypto wallet and crypto transactions are taxable.
Matrix/Element would be my preferred option, but it causes so many security or encryption related issues that it has scared off everyone I tried using it with. Nobody knows what to do with the incessant popups demanding to "verify" something or other. Nobody (including myself) knows why older messages often can't be decrypted.
Telegram is less secure than WhatsApp.
Threema is not free, which makes it difficult for me to ask people to install it. It's not open source either.
iMessage is Apple only.
So what's left besides WhatsApp?