Those engineers won the lottery. The prize was many people switching to a marginally nicer chat app. If you're looking to make the world a better place, choosing engineering based on this result isn't really an obvious choice.
To examine the equation in the "work for an existing SV tech giant" case: a hugely smaller impact on people than what the WhatsApp engineers had. And getting a job at Facebook or another similarly impactful place is still at ~ 1:100000 odds for the world's 20 million software engineers.
Only this time you'll have even less confidence that you're actually making things better and not worse. At least in the WhatsApp case they made Facebook have $19 billion less money in the end. And if they're really out to make the world a better place they can spend that in some Bill Gates-esque way. I guess we'll see about that.
