Erlang was picked for a particular reason. WhatsApp uses a prebuilt open-source chat solution, an XMPP server written in Erlang, ejabberd. This thing was first released in 2003 (old). Already a smart move to get started.
I’m pretty sure they hired Erlang developers to dig into ejabberd internals and optimize certain things. They didn’t just decide to become an Erlang shop out of the blue.
It wasn’t Erlang that was the initial right choice, it was using xmpp and ejabberd that was the root reason. Erlang just happened to be a consequence of that.
https://www.ejabberd.im/
I will contest your attribution of ‘smart’ with Erlang. These types of correlations are generally bias fitting. It justifies ‘smart’ being correlated with any and all niche languages, eg ‘so and so likes Haskell, so they must be smart’. No good.
It’s better to attribute ‘smart’ with the pragmatic decision they made to simply use a pre-existing chat server solution that already has the capability to scale. Harder to assess this as smart since there’s no ‘signaling’ here, you have to objectively assess if it was the right tech (which it seems like it was). Way less vanity in this assessment as opposed to what I already pointed out, how your Haskell or Rust devs must be particularly smarter, as opposed to say PHP or JavaScript devs who are considered dumber. I don’t buy it, I need to see more than just your affiliations.
So, I reject your initial post contending ‘The technology stack isn't even that relevant.’ It was precisely the tech decisions that mattered, and the right people to make such decisions. Chicken and egg scenario, I’ll concede that.
In any case, one does not simply pick a old chat server written in Erlang out of the blue - this decision was critical. How many over-funded tech teams would try to do this from scratch in Go? Plenty, and that whole team would easily be full of ‘smart’ people.