Another giver of +1
The simple answer is to limit the depth of the midsole. If you say that midsole cannot be more than 20_25mm then you know that the leaf spring can only have a limited effect. If you look at the shoes Kipchoge is preparing for his 2hr attempt they are maybe 50-60 mm thick. This would allow them to accommodate a much more aggressive spring than the one currently in the next %. If the 4% is acceptable what about the 8% or the 16%... Because all Nike needs to do is keep adapting the spring.
The new track shoe that Nike Elites are wearing has a taped thick look g forefoot to hide the technology. This is clearly not within the rules. Nobody knows what it is? I've already seen a shoe with a supposed microchip midsole that can measure plant and pressure. This is only the beginning.
A rule limiting stack height to 30mm, availability on sale to all who want to buy.