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Replicant Roy's soliloquy at the end shows that he has more existential awareness (humanity) than most humans as juxtaposed against the human Deckard. The point being that because we know life is so short that it is bittersweet. That feeling the warmth of the sun is the sweet and the coldness of the grave the bitter knowledge that soon we will die. So the meaning is that we should enjoy and marvel at the beauty of a single blade of grass while we can.

Since the entire movie builds to this final scene of the juxtaposition of existential awareness between Replicant and Human the Deckard character has to be human in order to be the foil. Otherwise the movie becomes meaningless gibberish.

So the movie concludes with human Deckard deciding to put aside his qualms and just love (the elusive unicorn) Rachel the woman who happens to be a Replicant (who as Roy proved can be as human as any human can be).