Is Christmas season coming sooner each year?
Re: your conclusion, I don't think you've considered that Christmas's expansion spoils the fall holidays of Thanksgiving and Halloween, which is odd given how obsessed our culture is with Fall. In fact it's probably worth a statistical analysis of Fall moving up; its strange that people agitate for fall starting in the end of August, when, at least where I live on the east coast, the leaves aren't even changed until the end of October.
You mention Christmas reaching an equilibrium, but I wonder if the better framework is a tragedy of the commons. We all benefit from a limited resource of a temporally bounded winter holiday season, but anyone who wants the Christmas endorphins can pillage the commons by setting up decorations earlier and earlier. As you mention, the digitization of commerce makes it easier for individuals to defect, since you don't need to rely on a brick and mortar location that might be under some pressure to maintain an appropriate seasonal theme (though this front appears to have completely collapsed, the Christmas stuff at my local Hobby Lobby goes up in September).
Pessimistically, I have to wonder if the desire for more and more Christmas is seated in deep dissatisfaction during the rest of the year. It's like with any other nostalgic salve; If you want to watch old cartoons, knock yourself out. But do it everyday and I might ask if something's the matter.
How did you write all this without mentioning the Grinch even once?