Not necessary, because the argument never compares commercial honeybees to wild honeybees.

Not necessary, because the argument is concerned only with whether inbreeding has contributed to the population decline. It’s not concerned with whether the problem of inbreeding can be solved in the future or how quickly it can be solved.

Not necessary, because the argument is based on the fact that breeding practices have “limited honeybees’ genetic diversity.” Even if the diversity does not “continue” to decline, it’s still the case that the practice have limited genetic diversity, and this limited diversity can still contribute to a population decline.

Not necessary, because although we know that viral infections and mites have “devastated bees in the past,” we don’t know whether those bees were genetically diverse honeybee populations. If it turns out that those devastated populations were NOT genetically diverse, that actually supports the author’s argument.

Necessary, because if it were not true — if lack of genetic diversity CANNOT make honeybees more vulnerable to adverse conditions — then we have no reason to think that the limited genetic diversity from breeding practices plays any role in the decline of the honeybee population. If (E) were negated, the adverse conditions (like the viral/bacterial infections) could simply have caused the the decline entirely on their own, without being aided by the limited genetic diversity of the bees.