I think the designers got themselves into a bind where they both really wanted to include the sacrifices element, but also didn't want want the adventure to be set in a lifeless, post-apocalyptic wasteland.
In order for Icewind Dale NOT to be a lifeless, post-apocalyptic wasteland, this can't have been going on for two years. But if it's only been going on for a period of months, the people of Ten Towns look psychotic for resorting to human sacrifices so abruptly.
The compromise probably has to do with removing the "no sunlight" thing. A year or two of winter is a region on the brink of total collapse. A year or two of winter plus 4 hours of twilight/20 hours of darkness per day equals total extinction. But that messes with the duergar storyline.
Another possibility would be to introduce the idea that the sacrifices to Auril are actually working, and that she permits the towns some relief from the cold and dark in exchange for performing them. But that introduces a level of moral queasiness and complexity that I'm not sure WotC really wanted to deal with in this story. As written, there's no evidence that the sacrifices are having any results, so morally the adventurers are in the clear if they decide to put a stop to them. If you change it so that the sacrifices are actually benefitting the towns, then the players have to choose between allowing human sacrifices or putting entire populations at risk. Which would be great drama at some tables, but isn't the sort of stuff 5E adventures tend to grapple with.