In my opinion the whole “Deckard’s actually a replicant” thing ruins an important part of the movie: the juxtaposition of Roy as a machine that’s a man vs Deckard who is a man that’s a machine. Deckard unquestioningly hunts and murders replicants without remorse, robotically going through the motions, at least until he gets to know a pretty one. Roy murders people in a passionate, angry way, but in the end decides to grant mercy to Deckard, someone he should hate. This act of mercy breaks through Deckard’s shell, and restores his humanity to him. If Deckard was a replicant the whole time, it ruins his character arc imo.