considering the profits in GMO corn, expecting farmers to sacrifice 15% of their growing space to heritage varieties of corn is like asking a wall street banker to sacrifice 20% of his profits just for the heck of it. Not likely to happen when greed comes into play. the dumbass monsanto fools could just include non-Bt. modified corn in 15% of their seed so that the populations of corn root borers would have a stable population, so that farmers would have no choice but maintain diversity. (resistance to glyphosate would have to remain, or when they drench their fields in roundup all of the heritage varieties would be toast, but that is another story)
Bt.-mimicing genetics in corn also kill the beneficial insects... whole genera of insects are becoming scarce. (seen any monarch butterflies lately? roundup on most of the tilled acres means milkweed is scarce as hell, too) and that is just a popular insect, the ones you don't notice are taking it to extirpation levels too. and butterflies, for example, don't have gigantic populations that can quickly evolve past the genetic bottleneck imposed by monocultures of GM organisms like the more resourceful pests do, since the habitat of Bt. corn roots is prevalent all over farm country, while habitats of the "good" insects are in rapid decline due to chemical farming.
I personally do not fear eating GMO food, but i totally despise their effect on the environment which is almost totally negative.