>Can't farmers still use non-Monsanto seeds like they could 100 years ago? My grandpa's small family farm has been doing just fine without for decades, after all.

Monsanto is somewhat infamous for suing farmers for planting seeds from plants that were accidentally cross-pollinated with Monstanto crops. So to answer your question, you can't use non-Monsanto seeds if any of the farms around you are using Monsanto seeds.