They don't have to, as long as more buyers do than choose to not buy the "hot" item after Christmas. In general it's a good thing to increase the size of your industry, as long as you capture some (not all) of that increase.

Also, I wouldn't bet on them being more likely to buy a competitor's product. Brand recognition counts for a lot, particularly with toys & other discretionary purchases. Some will, of course, but most people once "primed" will buy something close to what they initially had in mind.