There is the perfect situation and then there is the best realistic situation. The perfect situation for maintaining and expanding genetic diversity is probably

There is the perfect situation and then there is the best realistic situation. The perfect situation for maintaining and expanding genetic diversity is probably going back to every region growing it's own particular strain of a given vegetable, with people in those regions mostly breeding and maintaining their own strains. In the really old days, pretty much everyone in a small region was growing the same variety (ish) and was saving their seeds and the combined gardens of the community (village, what have you) were more than large enough to be a self sustaining population, while also being the right combination of homogeneous (to maintain good traits), and large enough to have enough diversity to allow for new traits and hopefully even protect against disease or changing conditions. But that's not really realistic anymore. If someone like the other poster, who has the space (both to maintain a large enough population as well as the distance from other gardeners who might be growing modern strains), the time, and the interest, they can indeed be doing the "best" thing from a genetic diversity perspective (as long as they make sure that their strain doesn't die with them). But those people are very, very rare. So the next-best option is to try and maintain the current genetic diversity we have, in a persistent way (as opposed to a static way in seed banks), by having breeders breed as many different varieties as possible. They will hopefully be following the dual mandate of maintaining unique traits while also allowing adaptation to changing conditions. Your average gardener, even if they wanted to, can't maintain any real genetic diversity because A) they don't have the space for a viable population and B) their genetics will constantly be contaminated by their neighbor growing Early Girl or something. So making sure that the already extant genetic diversity doesn't dissapear by supporting heirloom breeding is not the "optimal" solution to genetic diversity, but it is, in my opinion, the best "realistic" solution.