Keith .. to answer your original question "Could it be these were just genetically weak and the die offs were natural, not disease?"
The answer is "Yes"
It all depends upon how the genes of the seed parent and the pollen parent combine to create a new plant. Historically, less than one percent of rose seeds produce a viable plant. Large breeding operations will sow tens of thousands of seeds and only a very small percentage will produce plants that may be brought forward for testing. Many of those plants will not survive the second year, so the percentage of that initial crop gets even smaller. Of those survivors, many of the new roses will be weak plants or will not produce the desired results of the breeder. If open pollenated, the rose seeds may never produce a good rose.
However, if you view the history of roses, early breeders planted roses near each other hoping that a wandering bee would carry pollen to a rose where the genetics were such that the seeds produced "could" produce a viable rose.
Again, it was a matter of numbers. They planted as many seeds as possible to up their chances of creating a new rose.
Edited for typo
