> e.g. tomatoes have lost most flavor and nutrition over past few decades

can't speak for nutrition (nor do you provide support for that claim) but as to flavour, you clearly haven't been round long! Old fashioned tomatoes were shite I can testify (at least those available in the UK).

My grandmother said the ones from the 20s in the us tasted way better, but they didn't keep any time at all, which was fine because they grew or raised all their own food.

same story here. she also said that it was commonplace for people to have plants that originated from seeds from 'the old world' and brought over; maybe they were at that point closer to some genetic cousin.

I grow heirloom tomatoes in the United States, and they are spectacular. The tomatoes available in the grocery store pale in comparison to anything grown in your own garden.

This probably has more to do with the fact that supermarket tomatoes are picked before they fully ripen on the vine (often green). I also enjoy growing my own (Brandywine is a favorite), and letting it ripen on-vine to the point where it's probably hours away from falling off or starting to rot results in a much better tasting tomato than jumping the gun.