The standard "smooth out the fluctuations" approach gives an extremely low probability of running out of fuel during the winter at a reasonable cost, but only assuming sufficient fuel imports. Because of the loss of Russian gas and not having sufficient LNG infrastructure to make up it, Europe will burn gas faster than replacement when it's sufficiently cold, and if the cold weather lasts long enough they could run out of gas. So they're now using a more expensive and wasteful strategy of keeping storage closer to full, exactly to reduce the risk of people not having heating during winter.