I was able to convince my coworkers to stop working on a Python project (a simulator for a scientific instrument) once we all realised that we would have not been able to keep using plain Python because of performance. When the alternatives where (1) to force everybody to learn either C or Fortran, or (2) to rewrite everything in Julia, they asked me to provide examples of both solutions. After I showed them a few different approaches (pybind, cbind, f2py...), there was full consensus for Julia. We moved what we implemented so far (it was not to much, luckily), and so far nobody has regretted the choice.

The problem in using two languages is that you do not have just to learn a new language besides Python. You have also to figure out how to properly interface them. (f2py was considered the easiest solution, but we feared that the 1- vs 0-based array indexing would have likely caused many bugs.)