If a question _is really_ no longer relevant, then presumably it would not be very likely to receive new answers, making closing a sort of redundant concept. On the off chance that someone's circumstances led them to a situation where the answers such a question received _were_ relevant, then there's no harm in leaving it around.

In certain cases, stale questions that lose relevance can become actively harmful by drawing people to the wrong information, at which point we have a couple of options:

 - The question can be closed as a duplicate of the question that's actually _helpful_ in a similar search

 - The question can be deleted if it's causing people to trip over it, or advertising incorrect information or guidance.

The idea of putting posts on hold, later changing to a closed state is so that they can be actively improved so that they're suitable for reopening. If something is gathering dust _and_ making anyone happening by it sneeze, then it's probably something that should just be deleted.

If it's not harming anything, then there's really no reason to think much more about it.