The question at the heart of this is: what does the essence economy of the underworld look like? The underworld traps ghosts in the walls when they can no longer spend essence to stave it off; the other dangers like the ocean of fragments are more avoidable. So deciding how much Essence flows into the underworld and how various specters manage that resource will determine how many and how quickly ghosts get consumed.

The way I imagine this, from my reading of the mechanics, is that ghosts in the underworld mostly get essence from being remembered by the living and from consuming Castoffs...