That is not to say that you can't get a job and you might win the market by doing something increasingly rare in relatively high demand. Or you might not. Or you might do ruby for a multitude of other reasons but relative popularity better not be one.
As you said everyone and his sister knows javascript, JS devs are a commodity. I don't think anyone can expect to secure his future by picking up something like JS or Python since there are tens of millions of people learning it right now. So the popularity of the language means just that - everyone knows it. There are more jobs but way more people applying.
Nor is Ruby a secure bet, nothing is really. Ruby is niche but it still has a very strong community, there is almost nothing web related Rails and the community haven't solved. I think people overrate the importance of community size. Does it matter if JS has 10 million devs or 20 million? Not really imo. Maybe even 10 is better than 20, less decision fatigue.
That is not to say that you can't get a job and you might win the market by doing something increasingly rare in relatively high demand. Or you might not. Or you might do ruby for a multitude of other reasons but relative popularity better not be one.