I think that race rules limiting size to say 10.0 would not mean banning sales of 12.5 sails. You could always be using one at will. Of course, we may discover at that point that the market for such huge sails would shrink and maybe no one would produce them any longer. But that would be a different problem... after all, I just see the light again after years of good slalom materials being totally disregarded...
If we banned sails bigger than 10.0, there would be no races in most of the US, and elsewhere, nor would anybody make sails bigger than 11.0. Even lightweights use bigger sails than that.