From what I'm understanding with What is a lifehack? and Is there anything that isn't on-topic here? A lifehack is a solution to a problem, so as long as there is a question being asked, it's on-topic here.
Because of this, I'm getting the sense that we're never going to be able to say that a question belongs on another site, because even though there's an overlap it's on-topic here.
The problem is that the overlap is so large, that we're going to be promoting the cross-posting of questions between this site, and every other site on the Stack Exchange network (which is something that isn't encouraged).
The reasoning behind why cross-posting is a bad idea is that:
A question that is on-topic on two or more sites is too broad. A question should be to a specific community.
Having duplicate questions across the network creates noise.
Having duplicate questions across the network divides experts. Leading to differing quality of answers.
I know we don't want to send questions away if they're on-topic here, but can this site really sustain itself if 90% of the questions on every other site can be cross-posted here?
How much of an overlap can we allow, and at what point do we decide a question belongs in a specific community?