The reality is they save a lot of money by keeping virtually no slack in their system and not doing things like scaling up customer service during the holidays. If they lose some money in flight cancellations, it's still rational for them to operate like this. Plus the reality is they probably won't actually lose much - most people whose flights were cancelled will still end up taking Southwest because it's too expensive (or not possible) to book last minute tickets on other airlines. That means they just push them into the very end of Dec/early Jan when they have open capacity anyway.