The key differentiation is whether there is something of quality to maintain.
I have never put it into words, but this is about the process I use when determining when to migrate:
- Is the question off topic here? If so, continue.
- Is the question a "good question" worth saving? If so, goto #4, else continue.
- Is there "good content" added by other users (i.e. comments/answers)? If so, consider editing the question to improve and then continue, else close.
- Is the question on topic on another SE site? If so, migrate to that site.
If I am not sure about #2-4, I ask in the mod chat channel. While clearly home networking questions are off topic here, that doesn't make them always worth migrating to SuperUser.
Just the same as I don't want other sites to migrate any network question here, just because they are "on topic." If they are on topic and good question (or could be made good questions), then sure, send them our way.