Earlier quoted context omitted.
WhatsApp is an ok substitute for Email in my opinion. Sure, it is not open source and but federated. Signal, Wire, Matrix, etc should catch up in the next years. What do you need from Email that a messenger lacks?
I guess you only use a cellphone then. Whatsapp desktop/web is terribly slow and requires your cellphone to be on at all times. Also, you cannot have whatsapp desktop/web active on more than one computer at any time. Also, I do have several identities (work, leisure, even one-offs for website registration/communications) and no, I don't want to give my personal cell number to everyone.
They have a rather sound reason for the cellphone requirement, though. It's because the messages are stored only there and the desktop/web application is just a gateway to your phone.
https://faq.whatsapp.com/en/web/28080002
This seems like a more secure way to do it than what the others do, which is to store the messages in their servers.