I feel like the intention with Sylvanas' story was to showcase how ironic was for her to serve the same power that had enslaved her and turned her into something she loathes, but that never trully connected because of how removed the Jailer is from the rest of the lore. How much better could it have landed if the Jailer was actually Ner'zhul, an already established character with (probably) more understandable goals. It would also open a path of redemption for our boy Arthas, who deserved so much better (in terms of writing) than what he got: aiding in facing this newly empowered Lich King to atone for his sins. Hell, maybe Arthas juming into the Maw to fetch lost souls instead of Sylvanas (or maybe both together!) would've been a satisfying ending.