Besides, I bet for ultra-elite marathon runners, the bulk of the shoe would outweigh the benefits it provides. You're talking about people who wear super-light minimalist shoes because it gives them an advantage.
There already is a lot of assistance in athletics, not even limited to footwear. For example, running tracks have 9 lanes nowadays, even though only 8 people start in sprinting events. Reason? The innermost lane is softer, for middle and long distance runners; the other 8 are harder for the shorter distances.
On the other hand, it always surprises me that, in athletics, competitors wear numbers tied to their shirts with safety pins. Especially in the 100m and 200m, the one on the back of the runner must cause significant drag.