You wouldn't print the buckles and everything, just like the buckles aren't in the molds that are used today. A boot wouldn't be all that hard to print because its plastic, which is the issue with printing liners. A material that is usable in a 3d printer and feels like what they use for liners right now is either very hard, or impossible at the moment, we'll have to see where the tech goes.
I do think some iteration of this is definitely in the future for boot tech, as an upper end model of boot (i.e. DaleBoots), and will begin to make its way into cheaper and cheaper models.