i 100% agree with you. No one knows what unfair is - that's why the rules need to be changed to something along the lines of , "No one can compete in a shoes that hasn't been out on the market for XX amount of time (3 months, 6 months etc)."
The reality is is one guy has the vaporfly 4%'s and no one else does - that's not fair. The reality is the 2016 Olympic marathon wasn't fair (thankfully most of the top male athletes were Nike anyway) It's a huge advantage, proven in scientific papers. Our sport is one that will DQ Ezekiel Kemboi for cutting a fraction of an inch off in a nearly 2 mile race but allowed Kipchoge, Rupp and others to potentially save more than a minute thanks to their footwear.
The rule needs to be changed.
Your proposed "no-prototype" rule would be a radical change. As I've noted before, at one point most track shoes were custom made by cobblers, and the first 4-minute mile was run in a pair of very high end, very light shoes that were not designed to last very long. Also, using pros as guinea pigs for development is a big part of the value proposition for brand even deciding to sponsor runners. Nike's slogan for as long as I can remember has been "engineered to the exact specifications of championship athletes," and most other companies use a similar approach, albeit with a lot less R&D money. I don't think we should have a rule that further decreases incentives for shoe brands to invest in the pro side of the sport. Finally, even if we had this rule, it wouldn't matter because as you noted, it's not commercial availability that matters at the elite level; it's the fact that people are locked into sponsorships.
As for the unfairness, that's just an issue of degree. Keep in mind that: (1) all racing flats increase running economy vs. running unshod, and (2) not all racing flats do so to the same degree. Therefore some runners have always had an equipment advantage. There was a study on Adidas Boost foam demonstrating that it increased economy by 1% vs. a control shoe that was identical except for the foam. Was that unfair? Where between 1% and 4% does the unfairness emerge? (Also, as an aside, even if 4% is the correct number in lab tests of economy, it's quite clear that that the VF is not translating into a 4% improvement in performance. One of they mysteries of exercise science is that interventions that have been repeatedly shown to improve performance do not "stack" on each other when you use them all together, which may suggest that a lot of these interventions are ultimately influencing the brain, even if their benefits initially appear to be related primarily to particular peripheral effects. I'm not aware of the VF having been "stack tested," but it would be interesting to see if the VF+beet juice produces the sum of the running economy of either alone. My bet would be it doesn't.)
I'm not saying that shoes shouldn't be regulated and that the 4% isn't potentially a bridge too far. I'm just saying that what's happening now isn't a revolution. It's a continuation of something that's been happening in the sport for a very, very long time, which to me suggests that the "unfairness" isn't so obvious or something that we can just take for granted. The proper response isn't obvious either, which explains why the IAAF is moving slowly.