I’ve been thinking about the issue of lost genetics in plant breeding. Inbreeding rarely works out well long-term.
There is a lot of ruin in a genetic line; however, because it takes significant inbreeding to create a consistent cultivar that will maintain the same morphology, much of the genetic variation is necessarily eliminated.
As heirlooms are generally created in a single location, they are adapted to thrive in a specific locale with specific weather conditions, pests, soil, etc.
This means when you buy those seeds and grow them in your garden, they are not necessarily well-suited to the space.
Ideally, then, you would plant a wide variety of seed lines, then select for survival/vigor for a couple of generations insuring adaptation to the bioregion, then select out specific qualities for further inbreeding, insuring maximum adaptation to your garden.
Hence, Landrace Gardening.