There's a middle-ground stance that's pretty common in the Elixir ecosystem: five or six libraries get written to solve a problem over the course of a year or two, people play around with them, and then a common set of primitives gets factored out of those libraries and made into an interchange library that all those libs support.

That interchange library can then be integrated into the Elixir stdlib—that's what happened with DateTimes in Elixir—but there's no reason it needs to be. It works just as well to just keep the interchange-type library as its own tiny little dependency that all these libraries pull in.