but they need to have died for that. I know plenty forums that have threads that are running for months or years - because they've never gone for $threshold weeks without a post. (and in general I tend to think locking after time is bad anyways, or at least should be able to be overridden)

Whereas even within the days where it is still possible, you are not going to see "oh, this HN/Reddit discussion from last week is still active" because it doesnt get ranked anywhere based on that.