Blade Runner is about memory and mortality: memory as the fountain of genuine life; mortality as the completion of memory.
A replicant begins full-grown, with not a birth date but an incept date. Rachel's memories of childhood are implants, not lived by her but merely received; her life is not genuine. Leon has his precious photographs — some secondhand, but others of his companions: assertions of Leon's own memory, fragments of a genuine life. Roy recalls the magnificent things that he himself has seen offworld, things never seen by Deckard, things that will be lost when Roy dies (by design) only a few years after his incept date.
Roy's mortality, like that of all replicants, is more a brutal termination than a proper completion. Humans acquire their memories slowly, naturally, almost leisurely. Their lives accumulate. Humans live; whereas replicants only briefly exist and must long for life. Even so, the plight of the replicant is of course a metaphor for our own plight.
Although the humans seem to have the better of things, the point of the film is that we do not. Our lives may be genuine, but they are no less ephemeral. Death, whether after four years or eighty, is still death. What, after all, is the final line of the film? As Gaff says of Rachel: "It's too bad she won't live; but then again, who does?"
Now, imagine how stupid the movie would be if Deckard were a replicant, too. The photos that adorn his piano — those old, sepia-toned photos, presumably of generations of his family — become as pathetic as Leon's. More pathetic, indeed meaningless, for at least Leon's are of his real friends. If Deckard is a replicant, we would have to conclude he is precisely as deluded as Rachel. He, too, is one of the latest models who don't even know they're replicants.
Which means that his entire history as a replicant-killer is false. Which means that all his world-weary despair is false. Which means that the heart of the film — the tension between the tragic replicants who cherish life (yet kill) and the degraded human who takes life for granted (while killing replicants) — is false. Which means that the end of the film — when Deckard chooses not to kill a replicant — ceases to be a moment of redemption for Deckard, and even in the "not-as-happy" director's cut becomes no more than the happy escape of two misbehaving robots.
Gaff's line means nothing if Deckard is a replicant. It should assert the equivalence, really, of replicant and human, at least as regards the brevity of earthly existence. The first part ("she") connects to Rachel, the second ("who") to Deckard. But if Deckard is just another Rachel, Gaff's line is emptied. Its second part becomes a mere aphorism, disconnected from any actual protagonist. If Deckard is a replicant, then he no longer carries the role of human in the film. The themes of memory and mortality become abstracted. Indeed, the themes are undermined, since suddenly what we thought was human actually is not. So what then is real?
What then is the point?
The movie becomes nonsensical if Deckard is a replicant! Either Deckard is as new as Rachel and everything about him is a lie, or he's been around for years, pretending to be human, some sort of stealth replicant deployed by Tyrell for — what? To prove what? You would have to deny everything Tyrell says and does, and go searching for grassy-knoll winks-and-nudges, to believe that Deckard is a deluded replicant. It doesn't matter what half-baked cleverisms Ridley Scott intended; it doesn't matter how many dopey unicorns clutter the film. If you believe that Deckard is a replicant, you are colluding in a cheap twist and have ruined the story.
I am not normally one to dismiss the intentions of the artist; but if we are meant to conclude that Deckard is a replicant, then Scott's intentions are bad, very bad. Say it with me: "Deckard is not a replicant." Nothing in the movie mandates that he is. Say he is not, and you will properly see Blade Runner for the wonder it is.