Ironically, this was much the case for the Roman Kingdom/Early Republic and the Late Empire, when slaves were a rare commodity. During the history of Rome slaves gained a certain number of rights (such as the peculium, to possess semi-personal property) which left them better off as say, many slaves of modern Empires.
It wasn’t even that rare - especially in the Late Republic - when slaves would often buy their own freedom or they were freed for frivolous reasons (again, unlike many modern Empires). In fact Augustus had to pass some laws to limit the manumissio of slaves to somehow keep the number of Roman citizens under control (of course, he claimed it was to preserve the tradition and morality of old).
To be fair Tacitus was one of the historians that tried to be as dispassionate as possible in his works although he lamented the good old times of the Early Republic.