I do think there needs to be some middle ground. Right now it's vastly simpler to replace all your slaves with higher level ones you capture. there's little benefit to keeping the starting ones, except the uniques that have the special traits. Basically the only slaves i bother to level up and train are the uniques, and whatever decent Dragonkin I can get my hands on.
Wondering if the game needs something like "If you level a slave up 4 levels in your employ, you get to choose an additional trait", or something along those lines. Something to reward keeping slaves long-term.