I really don't like the ethical argument. Not just because it regards a setting which is easily changed, but because it really shows a shortage of sound ethical reasoning. Slavery is bad when the alternative to slavery is freedom. That is not the alternative for captured goblins.
my problem with the ethical argument is simple. different societies have differennt ethics. Why would two separate dwarf societies on opposite sides of the wworld have the same ethics. 90% of humanity see cannibilism as digusting. But that doesnt mean it didnt happen in some cultures. I'm sure some dwarven societies would see slavery as ok.
Ever heard of the
wisdom of repugnance? It's a crock of shit, but it's got traction with a lot of people, even if they don't know that term for it. I'm sure you've seen it in action. People who already feel that something is repulsive will often reflexively reject it, needing no better argument than their knee-jerk disgust.
Consider how fortresses come into existence. A handful of dwarves is sent out into the wilderness to find a new place to settle. They come from an older fortress, and maintain contact with them at least until they're mature enough to survive on their own. It's not entirely dissimilar to the
runner method of plant propagation. Each new fortress is merely an offshoot of what came before.
The point is that many fortresses, no matter how widespread in the world they are, will share a common point of origin. Each will have members bringing with them the old traditions, attitudes and prejudices into the new locale. The initial dwarven anti-slavery attitude is so strong (a capital crime!), that those indoctrinated with it likely feel a deep and personal loathing for the idea, much like many people feel about murder and rape. It's not just a crime you don't want to get caught doing, it's a Big Deal. All it takes is for this idea to catch on in the origin point, and it will propagate all over the world. So while there are almost certainly occasional dwarves that think maybe slavery isn't such a bad idea, they are always going to be surrounded by people that flatly reject the idea and may destroy them for putting it forth. The only reasonable exception to this is if by some fluke, the first mountainhome winds up being pro-slavery before sending out any runners. And then you wind up with most dwarves being pro-slavery, not just a few isolated fortresses.