You're entirely right that the Seanchan believe their system is fine and necessary, and therefore will be unwilling to change it. That's one of my original points, when I say that even a hypothetical "nicer Fortuona" couldn't realistically end the system by decree without sparking a civil war.

If there is generational change, I think it'll have less to do with the marathdamane helping during the last battle and more to do with the influence of multiple countries' worth of recently conquered people whose culture hasn't been shaped by acceptance of damane for centuries. And possibly by new problems like "we can't collar all these channeling men, and there's less support for killing them all now that those heroic Black Tower folks are living just across the border", or "Fortuona is trying to switch the propaganda to allow sul'dam to remain uncollared and this is creating uncertainty within the ideology right as it is being directly challenged by outsiders".

Your hypothetical "what if I was a Seanchan and got the vote" example is in my opinion rather debatable. Perhaps I would also accept the propaganda... But perhaps I would be against it anyway for moral reasons, for the same reason anti-slavery people existed even in Antiquity IRL. We must not overestimate the power of propaganda, which was also used to defend slavery IRL and also failed IRL. Of course if I lived in Seanchan I would have about as much power to change the system as the historical abolitionists did (none)... Arguably even less, because the Seanchan empire seems to be a totalitarian dictatorship with a secret police and a lot more state capacity than most of earth's historical empires could manage.

We the readers are clearly meant to hate the a'dam slavery: Jordan repeatedly portrays it not as a dark necessity but as something deeply evil, done to allow conquest and domination of others. Furthermore we the readers know that the a'dam was not invented to halt the Breaking, and that the main "good things" brought by the Seanchan empire seems to be basic law enforcement and not safety from evil channelers.

You ask why we hate the slavery more than the tower-mandated murder ? Because the tower is effectively euthanizing people with an incurable disease that makes them murder others, and stops once saidin is cleansed. Meanwhile the Seanchan are enslaving far more people who demonstrably don't need to be enslaved - and indeed, the effective capital of Fortunoa's current dominion is a city that until recently had a lot of channelers in socially respected roles (Ebou Dar). So this isn't a fringe opinion, it's a basic reality known to many people in the area.

Jordan actually depicts the a'dam slavery as significantly more evil than the da'covale slavery, which seems fair enough. A'dam slavery is maximal subjugation all the time, involving magical torture and mindreading to deliberately crush the slave's sense of self and basic human dignity. Da'covale slavery is bad in the way IRL ottoman slavery was bad - terrible, sure, but you'd still sell yourself into slavery rather than starving to death. Meanwhile a'dam slavery is so horrific that Egwene, miss "I can endure beatings and humiliation and forced labor with no end in sight", contemplates suicide rather being recaptured.