Earlier quoted context omitted.
That was probably the biggest impact single decision for WhatsApp technically. 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…
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.