@wejo: Really great question. When thinking about regulating shoes, there are two really important, valid questions that often get packaged together in a single debate, and that is: 1 - Should we regulate footwear? 2 - If so, how?
Setting aside the first question right now (I can touch on both sides of that debate after), a good answer to the second question - how to regulate shoes - has a couple important requirements. First, we need to create an operational definition of a "fair" shoe. The old regulations had no clear definition of what constituted an "unfair" advantage. Second, this definition obviously needs to effectively control what you're seeking to control (in this case, the extent to which a shoe can augment performance). Third, this definition needs to logistically feasible to enforce. Fourth, you would ideally have a regulation that is somewhat "future proof" or forward looking, rather than something reactive (simply banning things that come out). This is important for the appeal and generalizability of the sport, but also for the companies, as it opens the door for corruption and bias within WA and stifles their ability or desire to develop new products (always having the fear WA might react against your new product if it's "too good").
I felt that limiting the stack height seemed like an elegant, straightforward solution that ticked those boxes and also afforded a compromise to allow for innovation. It's essentially an engineering design constraint - a geometric black box that defines what a shoe is on an athlete (literally, the space on which an athlete that a shoe can be), and encourages optimization within that space. If you think of the shoe as a nebulous addition to bottom of a person's foot, it simply regulates the size/extent of that addition.
To choose a handful of mechanical parameters on which to regulate (resiliency (i.e. energy return), stiffness, material composition, etc.) would be an enormous burden for World Athletics to enforce, and it would also make the sport somewhat intractable, as it would lose the universal appeal. This is the route swimming took with suits, and they set limits on the buoyancy, permeability, and composition of the suits. They have to be approved by a FINA lab. The stack height limit is operationally appealing, as the eyeball test grossly keeps it in check, and you essentially just need a pair of calipers to measure or enforce it. A good test method/device is still lacking from the WA rules to make that exact measurement explicit, but hopefully that's being worked on. Moreover, choosing which mechanical characteristics to regulate, what the actual limits are, then writing the test methods to do them would be contentious. It could be effective at limiting the effect of the shoes (second criteria above), but I thought it would be an overly complex band-aid. Moreover, it would be really tricky to nail down any sort of material composition requirement (apart from testing foams
As a geometric constraint, it serves several regulatory functions. It limits the complexity of the shoe architecture and the extent to which that shoe can afford a performance advantage. With less "space", literally and figuratively, to work with, the extent to which materials and architectures can be arranged to create something substantially beneficial is limited. For example, with the plates, a more and more constrained midsole would limit the variation and shape of those plates. At some point, if it was thin enough, it may not even be beneficial anymore to have a plate there. On the flip side, the thicker and thicker the shoe becomes, the more complex the architecture within it would likely become to stabilize it and realize the advantage (the AF gave us a taste of this. What would that look like without a 40mm limit?). Moreover, it effectively limits the amount of mechanical energy that can be stored and released for given stiffnesses and compliances (a 40 mm spring stores twice as much strain energy as a 20mm spring). However, it does allow for optimization within that space. I'm not at all opposed to shoe enhancements - EVA foam is quite mediocre, if we could have foams that approached 100% energy return (i.e. perfect springs), that would be awesome in my book (there's a lot to be said about what potential health benefits may be afforded by better foams) - I just would advocate for drawing a line so we can reach an asymptote in the effect of the shoes on performance eventually (and preferably soon). Optimize within our definition of a shoe.
However, when thinking about stack height/midsole thickness in relation to performance, this is important: it is NOT a sufficient condition for an advantageous shoe, but it likely is a necessary condition. That is, a thick shoe is not necessarily going to be beneficial (the Hoka Bondi is probably not going to be your dagger of choice), but an advantageous shoe is likely going to be necessarily thick (you're probably not going to get 4+% economy improvements with a 25mm shoe).
As to what that stack heigh limit should be, that's definitely an open question whose answer will inevitably be arbitrary. However, that's true of most rules in sport - they're arbitrary definitions that provide a framework for us to understand the competition and performances. Why are soccer pitches 90-120 meters long? Why not 125 meters? Those were the rules that FIFA adopted in 1904 because it best suited what they understood as a reasonable pitch that suited the game in the form they wanted to propagate. I felt less strongly about what exactly the limit should be, but more that it simply be defined so we can start working within it. However, for the reasons described above, I would have leaned towards a more restrictive limit, like 30 mm, as it would have brought down the proportion of that performance that is afforded by the equipment. Also, the more and more space that a shoe can be, the more and more intellectual property becomes an issue. So, the greater the space in which to develop patentable architecture, the more and more brands have control over their effects. We see this with Nike's patent on the shape of the plate in the VF. As that space grows, so too do the possibilities to control advantageous designs within it, further taking the performances out of the hands (check, feet) of the athletes.
Finally, the more and more complex the shoes become (afforded by a greater and greater design space), the more and more they become fragile to the context in which they're used. Things like road surface, turn radius and frequency, weather conditions, even speed at which you're running will start to have distinctly different optimal configurations that would afford different equipment substantially different advantages. This exists grossly now (road flats aren't going to be great on a cross-country course), but you're not going to wear different flats because of certain types of pavement of the tightness of turns. Professional cyclists usually take at least 3 different bikes to a grand tour (a normal bike, at least one TT bike, and a climbing bike). And that's with tight restrictions on bike geometry. Do we want that level of equipment selection to determine outcomes or be part of our race-day decision tree? Some may, but I'd prefer to limit that. I love running in the AlphaFly on a track. I hate running in it on roads. Anything that's a bit uneven in the surface feels like it throws me off for a few strides, so I'm seemingly fighting the shoe, whereas on the track I feel like I slip into great, undisturbed rhythm. It's fun when it works, but I resent that variability a bit (and would hate to assess a race on whether the surface is sufficiently smooth for it). Moreover, at what point to these architectures become something that aren't advantageous to begin with, but can be trained with to better harness their advantage? That's interesting scientifically, but I'd really lean away from having that as an additional variable with which we'd have to tango with (especially from a developmental standpoint).
I also hoped that with a reasonable stack height/thickness limit, you could get rid of the rules around plates (limit the number/geometry of them) and the rules on prototypes. My personal feeling is that if we're going to allow shoes to be 40mm, we might as well develop as best we can within it (and not worry about adding more inevitably contentious arbitrary determinations). Again, just a single standard to freely optimize within.
Moreover, at some point, we can always loosen the restriction in the future if there's an appetite for it. If something is worth protecting, I'd say it's more prudent to regulate first to protect it and relax accordingly in the future, rather than letting it play out unregulated and risk losing or permanently damaging it.
As for the first question of whether we should regulate footwear at all, that certainly is a valid debate. There is one camp that would say we should either be barefoot or have no regulations at all. I actually think from a strictly biomechanical perspective, this is theoretically sound (and intellectually interesting). We're a sport that has largely known footwear to be a component, so barefoot would be unreasonable for most. On the other end, as mentioned on the podcast, stack height is certainly not something that is: "If some is good, more is better". Running on stilts (even optimized carbon-fiber ones!) is likely not going to be the best way to cover variable ground on foot quickly (though it may be the case that that becomes one configuration that is optimal to one hyper-specific person or context). There is an optimality. However, like I described above, the greater and greater the extent of that addition to the athlete becomes, the more and more that it becomes fragile to specific contexts. It also would leave the question open forever in perpetuity - is that performance the athlete or the equipment? That's interesting and cool for some, but I'd suspect it would ultimately erode the appeal of running.
Walking the middle ground - limiting the extent of the shoes - will inevitably beg a definition that's arbitrary, but that definition will provide a framework against which to bring our efforts against our opponents, the clock, and ourselves into relief.