Everyone I know uses email. No one I know uses whatsapp. A couple of people I know use Signal. A handful use iMessage.
> But that doesn't matter, because in this discussion, the figure of merit is encrypted email,
Ok, let's consider one case where encrypted email is commonly used: reporting security vulnerabilities. Do you really think any of these would be a good medium for that? Do you see companies or other organizations putting a whatsapp username as the contact in their security.txt?
I do want there to be a more secure replacement for email. But most of the newer e2ee messaging systems can't really fully replace email.