It has to be at the expense of something else… unless you’re positing two candidates that are otherwise completely equal.
I wouldn’t consider yichus more important than factors such as middos/yiras shamayim (to the contrary; those are certainly more important), but those are often unknown (beyond what’s readily observable), so for practical purposes, the two candidates might be considered completely equal.
If you were deciding between two schools for your child, you would decide based on which school seemed to have a better chance of enabling success for that child. You would consider who the teachers and administrative staff are, as well as the type of students they attract, and of course, the track record each school has with children similar to yours.
However, you would not be certain which school would actually be better; unknown factors, such as how well your child will interact with his/her teachers, principal, and classmates could play into it, but you have no choice other than to decide based on the known.
Even if you don’t consider yichus to be of great importance, at least grant it as a factor. In my above example, you might decide that the schools have equal merit, and the determining factor might end up being something as relatively unimportant as closer proximity or slightly lower tuition.