I've been rereading the End Times books and... yeah that's not going to work. Especially given the one thing I had completely forgotten about, something that first brought long before the End Times, the Black Pyramid of Nagash. It was designed and built specifically to gather and store magical energy from the Winds of Magic for Nagash. Once he was resurrected, Nagash pulled Shyish from the Vortex and bound it to Sylvania before heading south to get his Pyramid so he could bring it to Sylvania to slowly absorb Shyish into his being along with all the magical power the Pyramid had gathered over the millennia. In fact between his victory in Nehekhara and the Pyramid's destruction by the Skaven in Archaon he was far more powerful than the other Incarnates, bordering on God-level (non-Chaos God) power. It was the destruction of the Black Pyramid that cost him much of that power, dropping him back down to Incarnate-level, shattering his plans to become a God and forcing him to parley with the other Incarnates.
The destruction of Usirian and usurpation of the Underworld was two-fold in purpose. The first was to advance his design to achieve Godhood but the more immediate reason was to quickly win the War of the Dead. You see the Tomb Kings do not have necromancy as vampires and necromancers do. Rather the Liche Priests summon the spirits of their dead back to their bodies. But they can only raise their dead. They can't raise the corpses of their enemies or the buried dead of other nations. Nagash consuming Usirian allowed him to stop the magics of the Liche Priests from summoning their spirits, preventing them from raising their fallen forces again and again. Certainly Nagash possessed far, far more magical power than Settra but I suspect Settra was a better strategist and military leader. While Nagash would have likely won the war anyway but Settra, with every Tomb King and their legions roused and at his side, would have made it a long, drawn out struggle for Nagash. Something he probably didn't want, especially if Vlad and the other Mortachs he sent to the Empire met with problems or someone, like the Elves, tried to unbind Shyish from Sylvania.
And that doesn't really address the matter that some don't like, the Incarnates. Nagash became the first Incarnate when he snatched Shyish, paving the way for the others. Well that and the fact his undying spirit had become heavily tied to Shyish over the millennia. In fact if I remember correctly every time Nagash had died his spirit would come to exist within Shyish, not entirely unlike Sigmar's imprisonment within Azyr. Though Nagash was able to speak to some extent with Arkhan and other lieutenants.
As a Tomb King player I didn't actually mind all that much the Tomb Kings being folded into the Undead Legion with the Vampire Counts, I was more annoyed that they got overshadowed by the vampires and that they only paid off that final Settra scene in the novels rather than the game books. Though Vlad's story was one of the best stories in the End Times.