Once a discussion becomes obsolete, the thread will usually die anyway. It's only when members keep posting in a thread that has run its course and posting in a way that is not civil or pleasant that it needs to be closed. Which is why Gavin closed it (and not specifically because it was obsolete - if it was obsolete, we would have let it just fall off the front page).
In fact throughout my entire time here at TPM, both as a member and a mod, the vast majority of thread closures are triggered by the fact a thread has descended in petty squabbling (although this was a lot more common a few years ago).