Honestly, the prototype rule should be scrapped, for three reasons:
1) Whether there's a level playing field has NEVER had anything to do with whether a shoe is a prototype. It has to do with who your sponsor is. Adidas athletes aren't going to run in Vaporflys, even if they can buy them on every street corner.
2) Part of the value proposition of sponsoring elites is having them run in prototypes. The brands get feedback from their pro runners and when the pro runner debuts a new shoe, it creates buzz for when the general public can get it in 6 months.
3) Elite athletes have worn prototypes and custom shoes for the entire history of the sport. Roger Bannister had a custom pair of ultra-light spikes, designed to last for only a few races, when he ran the first 4-minute mile. Pro athletes have been wearing plated racers for over 20 years, and many of those shoes were prototypes or custom. Asics has shoes made to custom lasts for their elites. In other words, the Vaporfly didn't suddenly initiate a new era of athletes running in prototypes.