I am not looking for anecdotes that it is possible to write large programs in a pure functional style. I know it is possible. But, given the high cost of the approach (training, new libraries, maybe a new language and even a new platform) I am looking for anecdotes that the approach provides benefits that significantly outweigh its cost. I have not found any.

BTW, "extremely large projects" are anything above, say 20MLOC. What projects of that size have been written in pure functional style? What large projects (>5MLOC) have?