Well, JS was originally competing with Java applets in the browser, but, like you said, fitness for purpose is pretty significant!

My point (or rather, the point of the parent comment that I'm agreeing with) there's a lot more than just the presence and characteristics of a standard library that determine how widespread a language becomes

That's overly dismissive, Node.js has the same issue as JS in the browser and does very well. Small core doesn't matter there.