A hierarchical system does not become egalitarian overnight true but laws are passed overnight that attempt at changing that system.
Mere declaration of abolition of slavery by Jasnah is not going to magically make the system egalitarian and slavery vanish. There will be resistance and there will be a lot of work afterwards to make it a reality. But it is a first step in that direction and a good one. Many such changes have happened in our world historically speaking by the passing of legislations first and then slow and reluctant acceptance by the society later. Take for example practice of untouchability in India, widow remarriage, women suffrage etc. I don’t know much about American history but did not one president with the declaration of his intention to remove slavery caused it to happen in all the country?
Not earned how? Don’t you think that this issue has been addressed in twok and WoR. When a group of former slaves show extraordinary bravery, skill and character and save the prince and next in line to the throne of a kingdom and his army and earn freedom in doing so. When the same former slaves turn bodyguards to the king save his and family members lives number of times ( from assassin in white at that who no shardbearer had been able to stop as of then) when the same former slaves turn radiants and become unparalleled military power that humans have not seen in centuries and not only become instrumental in the humanity’s efforts against survival, but are the sole beacon of hope for winning if at all.
It already, in-world, has put a big question mark and severely challenged the merits of the practice of slavery and underlying belief in the superiority of lighteyes over darkeyes that such system thrives on.
Further, another major group of slaves, parshmen who were believed to be of sub human intelligence, have risen revolted against humanity in a war which for your average parshman is about slavery and their treatment thereof.
They have also shown that they are not sub human, they are not unintelligent, they once belonged to a thriving civilisation in ways much more advanced than Rosharan civilisation of today. They have proved themselves to be military power again. Again challenging the belief that humans are superior to parshmen.
All these events that have been happening in the books are about slavery and subjugation of one species by another ( not unlike racial discrimination of our world) and injustice of it and the fight against that
Events have been happening but the dialogue that needs to happen due to these events has not happened yet. Or in-world amongst the scholars, the dialogue must be taking place but was not touched upon in OB.
But now, one year later in RoW I am glad that this dialogue is going to be at the forefront with Jasnah spearheading the campaign against abolition of slavery.