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.
The people without smartphones that I work with can use email.
I can get to my email anywhere, everywhere, whether I have a phone or not.
Of course that may be considered an anti-feature from the point of view of security, but it explains why IM apps can not replace email.