Why is Facebook/WhatsApp doing the tether to the phone crap? I have a few seniors in the family who have no need of a cellphone (stay at home most of the time). Everyone else in the family uses WhatsApp and these poor people are left out of the look. It is completely stupid as far as I can see. What is the phone requirement buying them?
Imagine a scenario where you were to get rid of the phone number and had sessions open on the desktop and the web. WhatsApp servers have no way of knowing where the phone number went or if it will be online ever again or if you continue to own the phone number, or someone else owns it. They need to route the msgs through that phone/number combination all the time, because it's the single source of truth.
Its security and privacy that's preventing them from providing the feature you're asking for.
If they detach individual user's identity from a phone number, may be then they can be a true cross platform (web, phone, desktop) messaging app.