Also, having Deckard be a replicant kind of ruins a potential theme.
If Deckard were a human, it would mean that all the replicants want is life, which is a very human feeling, and all the humans were heartless to their replicant emotions.
Deckard as a replicant means that all humans are heartless, and some replicants are, too.
So Deckard as a human = you got yourself a neat subversive theme. Deckard as a replicant = theme gone but OMG HE IS AN ANDROID