I was an intern at Mixpanel at the time (but not the author of this article or involved in the discussed rewrite). For context, the company was then My recollection of events (of 8 years ago): The Erlang endpoint was written a year or two earlier as someone's first Erlang project, in a tiny org with no Erlang experience. The endpoint worked well enough and people moved onto other projects in the mostly Python codebas…
In general language diversity at a company is a big negative. Obviously sometimes there are even bigger negatives that mandate overriding that rule--you aren't going to use Java on the front end or javascript on the backend, that would be madness--but you never want two languages in the same niche. That's a cost with no benefit. If you have some developer that wants to write something in a new (for your company) lang…
It seems madness is pretty common.