Hmm, I haven't been following the details of the case, but this prompted some digging. It looks like the suit is not seeking a court ruling on legacy admissions, but that the group behind it is advocating for elimination of them. They have argued that eliminating legacy admissions would do a better job of ensuring diversity. Harvard is arguing that they need legacy admissions to get alumni financial support, and that affirmative action is being used to offset the discriminatory effects of legacy admissions.
So while the court is unlikely to force elimination of legacy admissions (no one has asked for that directly, but it is possible that they find some reason to take it to that), they very well might address it and whether or not they believe it to be viable.