That's an interesting point. Do you think your friend was able to deliver at such speed because of the familiarity with Erlang? Or was it so, that even with a lack of familiarity, the other advantages of the language were overwhelming? Also, may I ask what kind of application was being developed?

I also wonder how we can define popularity better. Does that mean industry acceptance, number of GitHub repositories, number of StackOverflow questions, number of libraries, developer satisfaction surveys, etc. If you choose different metrics here, different languages and frameworks will appear popular. Svelte for example seems to be on top of some developer satisfaction surveys, but React is still one of the number one choices for frontends. C is the industry standard for embedded, but developers often do not enjoy programming in it.