> Why bother investing in the languages with a much smaller market share when the big boys adopt good bits and pieces from them constantly?

This won't always apply, but sometimes (many times?) there's a software endeavor that permanently feels on the margins of a programming language's capabilities, whatever that means. You may forego market share in order to be 5 or 10 years ahead of current practice. In other words, the mainstream isn't converging to a stable F# / Scala / whatever. It's lagging behind. It's not the last time that a mainstream language adopts a feature that has been available for years in a more fringe ecosystem.

In practice, I'm sure the most important thing is humans and their skill, so mainstreamness matters. But if you can somehow be less sensitive to that, you may reap a lot of benefit from using Foo before it was cool.