Interesting read; not a lot of universal truths about re-writing. Which is actually a good thing because there are none. The decision to re-write is highly dependent on the context in which it is being made.

For example, a large legacy application in operation for over a decade with a huge volume of customer data is completely different monster from a smart phone app. Re-writing it in another language is a pain in the ass and if you're really, really successful nobody will notice the effort you put in. It won't make you any money or friends.

Still, there are times when it's just necessary. Even large legacy applications can be replaced given the right parameters. You can write an interface layer to the old code and extend it in a new language. Write an exhaustive amount of unit tests and acceptance tests against the legacy wrapper API and slowly replace the legacy code behind it with new code that is better written, faster, and more reliable (or what have you).

I think the most salient point of this article is to know what you're re-writing. Good acceptance tests and functional tests will help A LOT in making this decision.