It's debatable whether the final confrontation will be against Cersei. One of the crucial points of the whole storyline involving the White Walkers is that Westeros (and Essos for that matter) should unite if they want to stand a chance as the Others don't give a damn about noble houses, kingdoms, races etc. If most of them work together except Cersei, well her position will by default become highly unstable. While the biggest threat Westeros has ever seen in 8000 years is upon her kingdom, she doesn't do anything. So much for being the protector of the realm.
There's something wrong either way with the possibility that they first successfully deal with the White Walkers and then they just resume the game of the thrones.
That many people will die is likewise a given, but I kinda find it stupid that that would happen just for Cersei and that counts double for the dragons. It has been confirmed that the whole wight hunt, and thus the manner of Viserion's death, is a fabrication of the TV-series. In the books they put a lot of emphasis on the importance behind the symbolism of 'the head has three dragons', something that doesn't really appear much in the TV series. So there's quite a reasonable chance Viserion will not die in the books. It seems they reduced this plot line in the TV adaption to Dany and Jon, making one dragon 'expendable', but in the books this situation is quite differently.
In any case what was the entire point of Dany resurrecting and nursing thee dragons and collecting all those armies if they are mostly gone before the final act? This would make a large portion of the story obsolete and for what? To even things out for Cersei?