I used to work at an OCaml company and it wasn't nearly as much of an issue as one might predict. You can (it turns out) build a very successful business even if there aren't a lot of existing libraries, or if the language lacks certain basic features like native multithreading (same with Python of course). I don't have a great model for
why this isn't devastatingly expensive, but it's probably some combination of
* Most existing libraries are kind of bad anyway so you're not missing out much by not using them
* If you write everything yourself you get system expertise "for free", and gaining expertise in something that already exists is hard
* You can tailor everything to your business requirements
* Writing libraries is "fun work" so it's easier to be productive while doing it