They should have called "What you Leave Behind" the "Perma-Death Episode".
Usually, when a major character dies on Star Trek, they just bring them back a short time later. Spock is regenerated by the Genesis device, Tasha comes in from a new timeline, Dax comes back in a new body and basically does the same job as before, Weyoun gets cloned again, Culber gets... something to do with space mushrooms, Shaxx just comes back and no one explains how, and then there's Data's journey over multiple Picard seasons (no spoilers).
Contrastingly, when people go down in the last part of DS9, it's quite final. Even the apparently indestructible Weyoun is supposed to be permanently dead.