Why not Swift? (I think I know the answer). Concurrency in Swift is not yet a solved problem, but libdispatch is quite workable (although not "elegant, out of the box" per the article). With the work being done in Swift for TensorFlow [0], I'd imagine in a year or two both the infrastructure and the ML portions of a product like Cortex could be written in a single language. [0] - https://www.tensorflow.org/swift
Swift is an interesting choice, one we haven't explored in depth. Out of curiosity, have you done any work with Swift for Tensorflow/what has your experience been?
We looked at Go early on but quickly dismissed it because with the move from Python to Go, it didn't seem like we were getting enough benefits to warrant the amount of work that would be required.