Haven't had moment to read the new story yet, but I have to say that despite Bolvar being the Lich King, I dont think he's an authority on what would happen. Bolvar is a man of faith who believes in the light and that probably conflicted with his new role, so to him it would be losing himself to death's influence. But... Sylvanas is already touched by death and is already darker just like most who were good in life and are now "Darker" in undeath and she'd probably not be consumed by it, just like DK Arthas wasn't.
Now obviously the Jailer nagging at you is a thing which her being allied with him would probably mean he wouldn't do to that her, but Bolvar couldn't have known that and because he couldn't have known that, I dont think he can know that even without that, that she'd have been consumed aswell.
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When you think about it, a lot of characters have borrowed powers. I think in The Guardian that it's also stated that mages might not be creating fire but pull fire from a realm of fire or something like that. Then you have druids, shamans, light users who are definetely borrowing power from other forces and Fel and death users who consume other's souls to gain power. And then there's Tyrande who probably doesn't even know how to breathe with how much hand holding Elune does for her.
When it comes down to it, the weakest perceived characters are the ones who really are strongest when it come to their own power, Warriors, Rogues, Monks and Hunters. Garrosh, Varian, Saurfang, Taran Zhu would kick everyone's ass one of those areas where power is nullified