If my memory doesn't fail in the 4th Ed almost-reboot the muls could breed. Maybe they need some special trick, or sorcerer-kings discovered any way to "polymorph" "cheap humans" into "expensive and valious mul slaves". Even this could be a reward for loyal minions because they enjoy longer lives.
We can't forget sorcerer-kings as psionic-masters, and the have learnt how to use psionic hypnosis for mind-controll, but also they are enough smart to notice if you behave as a toxic boss then betray is more possible, altough it was only for vengeance helping an enemy.
DS also should tell about slavery caused by the debts, or by adictions.
Spelljammer as the rest of the setting is like a building toy, after you buy the box you can different things. Maybe some players bought Spelljammer not to play this setting but their own mash-up adaptation of their favorite franchise. I warn the impact of Starfinder by Paizo could be potential great even most of people didn't notice. Maybe in the reboot there are space for mash-up version of other Hasbro's franchises, for example Rom the Space Knight, Visionaries or Inhumanoids. Maybe even a future crossover Spelljammer/DC. (Not the original characters, but a mash-up adaptation, something like the league of Justice within RWBY universe).
Nentir Vale could become a second Sigil, and the Astral Sea be used to mix Spelljammer, planar travels, pirates and (under)sea fantasy. It was designed with the goal to become the ultimate sandbox, allowing all future new elements could be added later, for example classes and PC races.