> The average developer cannot pick up an entirely new language in a few days. Maybe I'm incompetent, but it takes me a few months before I start to feel comfortable with a new language and its ecosystem, and a year or two before I feel like I really understand the nuances and know just where to look when something goes wrong.

I have to agree that in most cases few days are not realistic, but I'm convinced you could apply Pareto principle and state that you can get most things done with knowing just enough of the language.

A year or two to understand nuances sounds like not making a deliberate effort to learn about them and instead just waiting until you get that through osmosis by just working with the language. It works, but it's slower than deliberate effort to identify differences from what you know already and learning specifically about them.

At any rate, arguments like that neglect that most languages are very similar, and after learning a few quite different ones the differences with most others become largely superficial.

> I can easily find someone with 5+ years of full time Java development (or JS, Python, and Ruby), and that means they're going to come fully equipped to dive into a problem, using familiar patterns and libraries.

Yes, and no. You assume that all people using the same language use similar patterns and libraries. Assuming recent experience with Python or Ruby that might be close enough to true in those languages, but I heard that in case of JS the ecosystem is incredibly diverse.

Setting aside any particular language, any sufficiently large codebase will have its own idiosyncrasies that add to the entry barrier.

> I don't want to hire people to make them learn Scala as quickly as possible and hope that they're able to figure it out very quickly.

You seem to be optimising for "very" short time horizon. It strikes me as short-sighted. You also seem to underestimate how much knowing the JVM as the runtime environment gives you.

> If you're working with Scala (or some other less popular language), you're going to have less results doing these things, because less people are going to have hit your special problem.

There will of course be fewer results, but you seem to assume that has to mean you will regularly run into trouble. Numerically, most Google results are bogus or even misguided anyway. You might try to argue that having more people using given technology increases the odds of finding a good solution, but that requires relatively stable signal to noise ratio across different technologies. I don't think that is necessarily the case.

Additionally, Scala might be poor example for this one, as it is actually used quite a lot in industry. It is nowhere near as esoteric as Haskell, for example.

> There is no amount of amazing language features that make this trade-off worth it.

This is a very dogmatic statement that I strongly disagree with. It is ironic to read it on Hacker News, given the emphasis from Paul Graham on how much velocity he gained by starting his career (were those e-shops?) in Lisp and being able to iterate much more quickly because of that. It is especially ironic given your focus on short term earlier in the post.

> And it's not just true for languages, it's true for frameworks too. Using common stuff (language choice, framework choice, library choice, compiler choice, etc) makes it so much easier to solve the problems you'll inevitably run into.

Straying from the common path should be weighed carefully for sure. But never forget that if you are far enough from the lowest common denominator for which it has to optimise, you might actually get way more mileage by using something else. Right tool for the job and all that.