I guess I just find myself scratching my head at your posts. I suppose how I see heirloom vegetables is how I see purebred horses.
Back in the day (let's say 1800) some farmers wanted *insert desired trait here* so they found horses that closely resembled those traits. They bred these horses and kept records by establishing a stud book. They kept the desired foals that reached close to the desired outcome and took notes on what didn't work. They added new blood and crossed lines and even line bred and once they finally found the "perfect" horse that produced itself with high heritability, they closed their studbooks and did not allow new blood in, making the breed PURE. This process is how I view heirloom plants. After so many years of not letting new genes in, the plants are considered pure.
I totally agree with you that when, for example, someone in Jefferson's age was playing around with their plants genetics that THEN those mixed plants were not pure or heirloom, but once they essentially "closed" their books so to speak, what we can enjoy today is that heirloom they created.
But, I will say that, yes, the topic of heirlooms in the garden is a very debated concept.