I could be wrong but I think even if they had modern down sleeping bags back then they would still have a problem with them icing up. The big advantage today I think is that they can airdrop fuel to heat a place up very warm in order to dry stuff out. Just heating up a tent isn't enough when its really cold. Just makes it humid. You really need a camp to dry stuff out otherwise your just melting the ice and freezing it again, plus some. I read someplace that bags can gain 50g a day of ice in polar conditions, which is almost a pound a week. It was only one article though.