The thing that would worry me about niche languages is the amount of wheel reinvention you'd have to do. Not sure the cost/benefit calculation starts to look so great any more when you realize that your enthused elm developers will have to build a whole lot of stuff that you can just import in other languages.
FFIs can have warts, but in the general case library support isn't the obstacle people make it out to be.
Interviewing costs money—good tech interviewing particularly so. The less of the people you have applying to start with, the more money you save on culling then by way of tech interviews.