Chiming in anecdotally here: 50k ``real'' lines of Python and climbing. No problems maintaining it but I wrote it all for my dissertation. It's scientific/CAGD programming, -Not sure but it feels like that makes it easier. Pretty sure I need to re-write it to entice others in on the fun. Anecdotes aren't valid far from the experience, I am afraid.

I, along with one other guy, maintain a >500k fortran 77 code base (bit of C/C++ sprinkled in) at work. It's way cleaner than my home-brew python. Maintainable is about wise practices, not language choice.