Monsanto owns all of the seeds. Or at least plans to. All of these companies have too much to lose. Obama will continue to “flexible” like he promised.
Any evidence?
Given that dozens of large companies, and hundreds if you include the small ones, own seed patents, you're going to struggle finding any.
BTW, no one is forced to leave the old non-patented seed behind. However, as it was in the 60's when single cross hybrids found their way to the field, no one could afford to plant the old, low producing, seed anymore.
Those old single cross seeds are still there for those who don't care about efficiency and have money to burn. The rest of the farmers are determined to make money feeding the world and will be using the best producing seed available.
Farmers decide what seed to buy. Monsanto does not have some super secret mind control device. Monsanto is among the industry leaders, but farmers have many choices. Most large farmers will plant several varieties, probably from several different companies, each year. This is because different varieties are suited for different soil types, and because farmers are always experimenting to find something that performs better than last year’s varieties. Which is why average yield continues to increase over time.