If there were more coatings then there might be more options to what the clear choice may have been. Perhaps coating C could have garnered quicker results, such as coating A, but without the stress deaths, such as coating B. The merging of those two qualities would have allowed the choice to be made clearer. However, dependent on the number of resources and competition, companies often try to push products through as quickly as possible and therefore, running extra tests would mean more money and more time.
In addition, although the autopsy for the rats did say that the 4 deaths were due to stress, could it have been that the stress was exasperated due to the coating, that it perhaps heightened it? But even doing that testing might be time-consuming.

But I do agree with you — it is better to be thorough and know the ins and outs of why issues happened vs. just making quick decisions.