Actually, refactoring a somewhat larger internal project in Google to a different language+architecture served us as a pretty good code review + self security audit. Yes, there was some turbulence at the beginning, but in the end it turned out to be way better understood/tested project.

I believe every project should be re-written every 4-5 years, it becomes less and less expensive to maintain it on the long run. Also keeps workers morale and promotion cycle healthy.

Unfortunately, not all companies have resources AND structure to put that resources in refactoring something that already works.