The fundamental misconception there is defining the Borg as "them," as if the Collective were a race of beings. It isn't. It's a single core consciousness that holds billions of individuals captive as its slaves. You don't defeat an enslaving, tyrannical force by killing off its slaves, but by liberating them. That's what Picard did for Hugh and the other drones in his cube, it's what Janeway did for Seven and other drones in "Survival Instinct," "Collective," and "Unimatrix Zero," and it's what the Caeliar ultimately did for all the Borg. Which pretty much makes it a proven success as a tactic.