Scott is a hack who just wanted to end the movie on le tweest ending just for the sake of it.
Ford and Hauer were both enclined to think Deckard was human and like other anon said, it makes more sense in the context of the book.
You have a human character desensitized to violence and almost mechanically devoted to his task and being allegedly incapable to feel but show the most human goals, values and wants in the whole movie.
Making Deckard a replicant destroys this narrative for the sake of a "oh shit he was a replicant all alone!!" reaction, which is a bit less impactful in the long run.
This debate is pointless, just choose your headcanon, both versions can coexist.