Actually, it was TNG: The Best of Both Worlds Part II that hopelessly kneecapped the Borg, by so easily defeating them with the sleep command. Before anyone tries to come back with "but it wasn't so easy", let's imagine how many species the Borg have assimilated. Are we to believe that none of them are as ingenious or capable as humans? Puh-lease.
That and also the notion of giving the Borg personalities and individuality by establishing Locutus, and later the Borg Queen. The Borg were most effective when they were these nameless drones acting with a hive mentality. Once you reveal that they're acting under orders from someone, and that there is individuality present, you do kneecap them a bit. When we learned that Seven was part of a "team" or "family" that weakened things even more. The Daleks in Doctor Who were never the same after Davros was introduced, and the creation of the Cult of Skaro further weakened aspects of them as a race to be feared.
Had the Borg remained the way they were depicted in early TNG - faceless, no individuality, acting as a hive - the introduction of a Borg regaining her humanity as Seven did would have been much more powerful (especially if they'd kept it so that she was the only one as well - remembering the later Voyager episode in which several others were freed from the Borg).