Well, the show pointedly says that they (humans) are “cheaper than droids and easier to replace.” It sets that state of play up very specifically. Even in real life multiple tasks one human guy could turn around, need multiple machines for the different tasks.

Slavery eliminates the cost of building droids in the first place, and in Star Wars those droids would basically be humans that need more maintenance than the nutri-sludge (oil changes, repairs, etc) anyway.

RE: the transfer scam, I think it was just feeding into the idea that the Empire is too arrogant. It’s not a scam they’ve been running, it came with the P.O.R.D. and no one thought it through to its inevitabilities.