I think people's opinion of the true threat of GMO's is misguided at times.
the issue is partly of course due to the nature of GMO's themselves, but also partly due to the threat of food security it presents.
We used to have over several thousand genetic variations of just corn alone, each expressing genes that help corn adapt to the environment it wishes to grow in.
Now lets say we stop using those genetic variations of corn, and stick with one altered GMO product that has a static genetic expression due to strict control of the DNA replication. Now lets imagine if a significant amount of farmers decided to stick with their GMO seeds rather than typical seeds due to subsidies that provide incentive for sticking with GMOs. As more farmers convert to GMO corn, less farmers are planting corn with naturally expressing and naturally mutating genes. What once was a gene pool with thousands of genetic altercations to aid in dealing with a variety of conditions from poor soil, different precipitation, and etc, is now limited to only a couple hundred.
It's as if we limited a human's capability to mutate and adapt to surroundings. Imagine if because the world felt so highly of white skin, we all genetically modify children to express genes for only white skin. Meanwhile those who have this alteration living near the equator are suffering from sun burns and increased skin cancer to the inability to express genes for darker skin that is better suited for this environment.
Well at least that's my thoughts being both a Californian and biochem major.