It's rarely "just" a port. It's usually because the new language has better characteristics. These days porting can be a very gradual, as-needed process - partly because of thrift, another Facebook tool!

One of the big parts of becoming a professional for me was accepting that code has a lifecycle; code is written to make the business money at the time, but it's entirely normal for it to change and die as time goes on.

(That said, you should just write everything in Haskell and then you won't have these problems. When was the last time you saw a company port code away from Haskell?)

It's tricky picking a "best fit language" when you're not sure at project inception what you're fitting it to. Say, for instance, that you're developing v. 1.0 of an application in a new application space. Your CEO catches wind that another company is looking to enter that same space. She is also adjusting requirements to find a business model that works. You would choose a development stack that allows rapid prototyping and refactoring.

Once you win user-share in the space the application starts having problems with scalability or debugging becomes an issue or a myriad of other problems that come with success. Now you need to consider writing parts (or possibly the whole application) in a development stack that allows greater scalability or has built-in semantics for core functionality in your application, thus allowing less reliance on 3rd party libraries, etc.

The other factor is that the team who authors an application may have a totally different skill set from the team who takes over the mature application. The authors may have been VB developers with good SQL skills. Your current team has more people who have dealt with Haskell in large-scale applications.

There is some parallel to home-building. A homeowner does a room conversion for more space, more privacy, etc. The builders of the home didn't install tracks and sliding-panel walls for better room configuration. At the time, they didn't see a need. The homeowner now has a need, so he goes through a much more expensive (relatively speaking) building effort.

Could be simply that time uncovers better solutions to the same problems, which triggers the need of porting.

The problem with this is just how often the story seems to involve porting of solutions. Not necessarily refinement or advancement, but pure porting. With lots of debate/fighting on the syntactic properties of the solutions.

Bit of Column A, bit of Column B.

All software tends to unmaintainability, even if you don't touch it (libraries change/deprecate etc). Given enough time, the problems that you are experiencing, say, memory allocation in C, get solved by something else, such as garbage collectors, and the amount of time it takes to solve the headache in what you've got is longer than just rewriting into the new language.

Plan to migrate to a new implementation every time scale grows 10-100x. Scale changes the relative weights of the concerns in the tradeoff.