From an experience standpoint, isn't the fact that you'd have to teach people a new language coming in the door detrimental to the use of that language? Comparing a company that takes in people that have been coding Ruby/Python/Java/whatever for 5+ years, versus one that takes in people that don't have Scala experience but teaches them on the job, wouldn't the former move faster and generally have more experience, ma…
I think you're right in the short term, but long term you may be more productive with Scala. -Devil's Advocate.
However, what also counts in the long term is the structure of the codebase, and I'd argue that people with more experience in a language are better suited to build more long-term-viable codebases with less technical debt just because they know the language better. But perhaps that's simplistic.