[_bxg1]

Tor is probably one of the better cases because of its stringent security requirements, but there remains an oft-ignored tradeoff of losing all that refinement work (and the fact that you can't just flip a switch to make it happen). It's a cost/benefit decision.

[steveklabnik]

Totally; that's why it's not a wholesale re-write, it's slowly replacing old code bit by bit, or writing new code.

Which is the only real way to migrate any codebase of significant size to a new language.