Why Deckard is human
Deckard is human because that's how Ford understood the character, and how he played it. So did Hauer, who played a replicant opposite a human--he'd have made other choices if he thought Deckard was just like him; instead, he's trying to teach a human something about real humanity; something Deckard finally realizes.
Both actors went into the production with that understanding, and that's how they played their parts. Both disagreed with Scott's later decision that Deckard was a replicant.
Scott would have to replace his two leads' entire performances--not simply add a scene with a unicorn from what looked like a deodorant commercial.