^That's an odd interpretation. Plenty of characters killed in Whedon shows haven't come back to life. Leaving out those who died in finales, they include Jenny Calendar, Kendra, Tara, Buffy's mother, Doyle, and Jonathan. You could make a case for Darla, who was actually resurrected twice (first when she was sired as a vampire), but whose third death was final.
Also, it's simplistic and misguided to assume that all Whedon shows are identical in approach. All the resurrections we've seen in his canon to date have been in the Buffyverse, which is blatantly a fantasy universe where magic and a variety of afterlives exist and it's actually quite common for the dead to return in some form (heck, the whole premise depends on the existence of vampires). And in Dollhouse
, the ability of an individual's identity (or at least a copy thereof) to survive death by transferring to a new body is one of the main ramifications of the core premise that the show explored, so it's built into that universe in a completely different way. But Firefly/Serenity
is a science fiction universe that, aside from a sloppy grasp of astrophysics and terraforming, is fairly naturalistic. There are no aliens, no faster-than-light travel, no teleportation -- really the one fanciful element is the existence of telepathy. The closest thing we've seen to a resurrection in that universe was in the first comic, where a character Mal shot in the pilot turned out to have not quite died from the injury. Clearly the ground rules are different, so it doesn't follow that resurrection would be as feasible.