Yeah the 40mm rule is pretty much where the Nike theory comes from. It's just convenient that they chose the Nike height as the cutoff, there a good reasons, but it looks a little suspicious. Really the easiest rule is to have a stack height limit. It's not unreasonable to set it at anywhere between 25-60mm.
My completely unfounded theory is that if either Hoka, Reebok, Saucony, etc came out with high stack super shoes that starting beating Nike athletes then the big swoosh would be throwing a hissy fit to every governing body to get them banned, setting the limit to whatever Nike's highest stack height is. Conversely, since Nike made the first ones, they could lobby to have the rule built around their model. It doesn't even have to be explicit, it can be as simple as World Athletics knows that Nike is the biggest company in the game ans doesn't want to ostracize them.
As far as how tall is too tall, there's certainly a bio-mechanical limit, but there is also a soft limit based on currently available materials. They could make 50mm shoes with EVA but they would be impractically heavy. Not unreasonable to image a better material becomes available that makes 50/60/70mm shoes feasible.
My completely unfounded theory is that if either Hoka, Reebok, Saucony, etc came out with high stack super shoes that starting beating Nike athletes then the big swoosh would be throwing a hissy fit to every governing body to get them banned, setting the limit to whatever Nike's highest stack height is. Conversely, since Nike made the first ones, they could lobby to have the rule built around their model. It doesn't even have to be explicit, it can be as simple as World Athletics knows that Nike is the biggest company in the game ans doesn't want to ostracize them.
As far as how tall is too tall, there's certainly a bio-mechanical limit, but there is also a soft limit based on currently available materials. They could make 50mm shoes with EVA but they would be impractically heavy. Not unreasonable to image a better material becomes available that makes 50/60/70mm shoes feasible.