Since we all tend to live in bubbles to some extent, do we have any numbers to back this up (like, # of repos/commits in each language on GitHub, or questions on StackOverflow)?

I tend to view TypeScript similar to CoffeeScript — it brings to JavaScript some features from other languages that are convenient and preferred by a subset of frontend developers. It allows for experimentation in the language, and helps inform TC39 proposals.

In time, I suspect support will coalesce around a specific TC39 process proposal to add type support, it will graduate to stage 3 or 4, get integrated into browsers and Babel, and enthusiasm around TypeScript will wane.