[X] Like Cordwood, Stack Terror
Druchii lives are the seed corn. Killing them is killing the seed corn. They can recover the slaves and the monsters. They can rebuild the warmachines. They can repair the arms and armor. Yes, they can birth and train more Druchii with time and thus eventually replace those too, but... Higher-ranking Druchii are the single-most thing they most hate to lose. (Outside of maybe one-offs like bound daemons or artifacts, dragons or giant monsters.) They are the least expendable thing on the battlefield, because everything else the Druchii consider "Dammit that sucks to throw wealth and ammo down the drain, but we can afford that."
If we destroy their monsters, slave chaff, and stuff... we might make them back off or reconsider how they fight the battle, maybe. But if we kill them en mass, it... I dunno. Might enrage them. Might break up unit cohesion. Might hurt or humiliate them more even if they manage to retreat in good order; they'll be at a disadvantage. It might force them to change how they fight, perhaps focusing more magic on defending themselves rather than attacking, or perhaps using the monsters to shield themselves more. Or maybe it'll make them get more aggressive and attempt to break through in one big push, putting it all on one big gamble... a gamble that might allow us to break them in turn.