A crop that is not resistant will not reinvigorate a disease, it will tend to weaken it (in theory). The resistant plants are the ones that select for more robust forms of the disease. Just think about the super germs found in hospitals. The diseases are still present in fields full of disease resistant hybrids. The modification to the plants simply allows the 'resistant hybrids' to hold on long enough for a profitable harvest, but the plants are still infected and propagating the disease organisms.
Most of the diseases copper sulfate is used for (foliar fungal diseases) are are not the diseases bred into modern hybrids, so there is little arguement there. You have to know what disease ails your garden (if any) and pick a specific hybrid to deal with it. That tosses water on most people saying 'I grow hybrids to avoid diseases' (well, what diseases are giving you problems?......)
The hybrids are better than heirlooms arguement holds up for commercial growers needing to make the most profit from fruit destined for tomato paste and MacDonald's burger slices, but not for the backyard gardener in most cases.
I do have a couple of spots where Fusarrium or Verticilium hits plants every year, and if it spreads, I may have to switch to growing specific hybrids, but someone has been growing tomatoes in this dirt for 30 years and we have not NEEDED to grow hybrids in all that time.
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