Tom, this was one of the last settled "points" of this whole proposal. From the survey response, when combined, the votes for "unrestricted" and "limited by wheel size" was the clear winner. (It was felt that it was safe to consider these responses together, because effectively they are the same, since the wheel restrictions aren't changing. In hindsight, they should've been combined into one singular response option.) But yes I too absolutely worried about the "what if", and if we should try to take some sort of measure to save us from ourselves, and try to limit people from trying to spend ridiculous time and money to get some very minuscule amount of perceived extra gain. One possible "solution" that was discussed a lot was to mandate a max rotor size per max wheel size. Say something like 250mm max for 13" wheels, 300mm max for 15" wheels, and 350mm max for anything bigger than 15". There was two problems with this though. (1) One of the major points to all of this was to give people more options, and (2) trying to control what any singular person may want to spend on this stupid hobby is an even stupider exercise. So it was decided to just leave the rotor option open to whatever a person wants to put inside their wheel. If a person wants to spend absolutely ridiculous money to try and get 5mm more rotor diameter than their competitor, who bought all readily available, off the shelf, affordable items, so be it. You, or anyone else, is not going to keep "that guy" from spending money and trying to do dumb stuff like that. But any attempt that you make to try to do that, is just going to limit everyone else's options, make our rulebook bigger and more involved, and make tech an even bigger pain in the butt. You have to admit, the difference in performance potential between these two options is incredibly tiny.
To your proposal, as the committee researched the idea of limiting brake diameter by class (or even weight), it was found to just not be possible. There's just too large of a range of wheel sizes and weights in any of our classes to make one fixed rule like that for everyone feasible, and would've created some serious "winners" and "losers". Unless you want to open up wheel sizes, which is a whole other can of worms. As I said above, when you look at what pretty much any car has for stock brakes, and how much larger they could possibly go within their stock wheel limitation, the percentage "gains" are all very similar.