Reminder that all canon materials were written to be as edgy and dark and marketed towards teens and young adults. If the Dark Elves were really as self destructive, treacherous, and uncompromising as presented in the Army Books, then their society could not have survived 6000 years without falling apart, even with an immortal godking guiding it.
Lets actually look at their economy for a start. They can't be that prosperous. Sure, they use slaves for all the manual labour—which I should state for the record is awful, because if I don't someone will accuse me of being a slavery apologist―but honestly, slaves are terrible for the economy. Why? Because they don't buy anything. They don't buy food, or clothes, or housing, or wine, or art, or books. That's devastating for an economy—if the working class isn't buying, then the merchant class has no one to sell to. The only circumstance in which it is viable is when a) the merchants are in another country selling imported slave produced goods or b) you don't have enough citizens to do the actual labour work.
And that second point fits with the origin of the Dark Elves—they are refugees from the Sundering.
Additionally, Naggaroth is known as the "Land of Chill". It's very cold there. Think "Northern Canada" cold. They probably don't have a great agriculture industry, and they probably don't have a great diversity of foods to chose from—aside from the spoils of war, that is. The Sorceress was even surprised at the wine she was drinking, and her offer was for items of novelty—knowledge and experiences they can't get at home.
So what we end up with is a small citizen population who don't have to work (because slaves are doing it), but also don't have anything to trade or eat. Luxuries and trade goods must be in very short supply, and the only way to get them is through violence because Ulthuan is hogging all the good trading ports. I imagine most Dark Elves must be in the military, simply because there is nothing else to do. It's very similar to the problem the Eonir face, only their population are artists, not soldiers.
But if we can open up trade routes, create an exchange of resources between the two continents, then that equation begins to change. Raiders are out, because they make the trade routes unprofitable, and merchants are in, because they secure essential supplies. And those merchants will want people to sell to—and the small number of elites simply won't create enough demand or jobs.
It's like the American civil war. One of the major sources of tension during that time period was that the northern states were wealthier than the southern states, because slavery was slowly becoming unprofitable in the face of industrialisation and the growing purchasing power of the lower classes. The war may have become about ending slavery, but it started about protecting plantation owners profits.
Opening trade routes with the Druchii gives rise to economic codependence. We can't change their society by declaring war on them, and we can't incite a revolution either—but we can apply economic pressure, and change their society by providing financial incentives.
And yes, this is something that would take years, generations, to pay off—but it's the start to turning Naggaroth into a legitimate society, and not an angry, militant colony on the edge of the world. Yes, people are worried about how this will affect the Empire—but those levers go both ways.
I'm not saying to jump them onto the waystone project immediately—give them a chance to build trust first, to establish themselves. And if they do betray us, then they'll lose access to the markets of the Old World—forever. And that will cost them a lot.
Or we can disregard their offer, and maintain the status quo of the Druchii raiding who ever they want with impunity with no incentive to ever change their society.
