Will you print a whole custom shoe, or just a custom insert for the shoe? The insert gives you a great fit, while using more traditional methods for creating the actual shoe results in a better constructed, longer-lasting product, for less.
They are already printing shoes on multi-material machines. I've held them in my hand. They would certainly fit better than an off-the-rack shoe, since they are printed to your specific foot. (Sadly, not MY foot, which is why I couldn't try them on!)
They use a harder, stiffer material for the sole/heel area, and a flexible media for the insole and sides. The sides were printed with vent holes, as well.
The top and bottom materials are fused together, so it looked like they would hold together forever. I can't speak to lifetime durability, but they certainly looked more rugged than a leather stitched dress shoe.