Friction shifting is the best thing for winter cycling where it gets cold.
Another issue is that with indexed systems the pawls in the shifters and lube gets cold and they slow down and mis-shift.
My Ultegra brifters and Xt thumbies don't like cold at all... indexed dt shifters do not seem to complain as much.
You'd be the one to ask. Hypothetically speaking if the problem isn't the OP's cables, and is in the shifters would repacking the usually thick grease in the shifters with a marine grade light grease (the type used in fishing reels) be a better option since it is less likely to gum up and harden at lower temps?
Get a bicycle. You will not regret it if you live. --Mark Twain