How was it unsustainable? As far as I know they were simply competent. They charged $1/year, so had ~half a billion in revenue, right? They probably could've bumped that to $2-$5/year with similar uptake. And they ran it with ~500 servers and 50 employees 12 years ago, so could probably do the same with ~50 or fewer servers today.
They never charged everyone. I was on Android back then and never paid a dime. Neither did anyone I know who was using Whatsapp on Android
They're doing a lot more now, though. Voice notes; multi-way video and audio calls; e2ee. And they barely even charged $1/year. I never paid for it.
Whatsapp revenue was $10M and the cost of revenue was $52M, with total net loss of $138M/yr just before facebook acquisition.
[1]: https://www.sec.gov/Archives/edgar/data/1326801/000132680114...