Language choice I can perhaps agree with, but from what I've read, the biggest factor with Erlang is much more its runtime environment. BEAM + OTP is a very impressive piece of kit.
I don't know. Erlang is a "funny" language that can somewhat easily do certain things that would be much more time consuming to accomplish in a more mainstream programming language. The difference between building whatsapp in Erlang vs any other programming language is probably larger than building it in php vs any other mainstream language.
Converse: Can you imagine WhatsApp parallel universe serving with Node?
Yes :)
Success is more to do with user experience..
If you hit gold and scale you can almost always optimize the implementation later.
Iirc Slack's backend is PHP.
As a rule of thumb based on my own experiences and the opinions of more experienced engineers I've had the good fortune to work with, language choice is far less important than the quality of the team using it.
While I have no doubt that trying to build WhatsApp in a language that would be the wrong tool for the job (say... PHP) would have been fatal, I have many more doubts that choosing Erlang was the key enabling decision.