If it's so easy to deliver pests which target those crops then why hasn't any terrorist group done it yet? The reality is that it would be extremely difficult, perhaps impossible with current technology.

Farmers aren't stupid. Monsanto has no way to prevent them from using non-hybrid seeds. Farmers will plant whichever seeds they expect to deliver the highest ROI.

Grain production before pesticides was far lower. Back then famines were common rather than something that only happened during wartime. Don't ask us to return to the dark ages.