I'm not so sure about that. We use to plant nothing but, hybrids, now we use almost all heirloom seeds. Personally, I couldn't tell the difference in the yield.
You might not be able to tell in a gardening setting because in a decent year they are all good and a decent garden will supply a lot of produce, no matter what seed you choose and seed choice won’t matter much at all. You only have a small number of plants and but a lot of different varieties. You don’t weigh each veggie and keep records.
In a farm setting you would have 30K plants to the acre and 3 million plants in 10 acres. You have years of records and you know when a hybrid or a GM plant makes a difference. Believe me, no farmer would pay the price for GM seed if it didn’t pay for itself several times over.
If, for instance, you lost all your pepper plants to phytophera, you’d just live without the peppers and rely on tomatoes and squash. But if you have 100 acres of peppers and they get Verticillium and die, it is a real economic disaster. So you would be looking for seed stock that had a resistance to Verticillium.
Even in a SHTF situation, there are still people who know how to hybridize, my husband’s grandfather did it back in the 1920’s, it really isn’t rocket science.
Even the fruit and nut trees are manipulated, the hardy, native root stock is used and then the desirable fruit or nut is grafted to them. You get vigor and hardiness of the root stock and the traits and yields of another variety.
Mankind has been manipulating seed and plants since they realized they could grow them on their own to have a steady source of food. They chose their seed from the plants that did the best until they developed hardier stock.