Ask HN: Rewriting a Codebase in an “Old” Language?

It's a common theme for people to discuss the success of a rewrite from one programming language to the other. Usually from an old language to a hot new language.

There's always a number of benefits claimed, such as less code, faster, better organized, etc.

My question is: how much do you think it has to do with the sexy new language, and how much is it due to the act of rewriting?

What if you take that sexy new code, and then do another re-write back to the old language? I bet there would still be a corresponding improvement again, even if a minor one.