As an insurance company can we assume their response is based upon statistical evidence?
If you wish to dispute that view then it may be worth checking their data if you can find it or simply asking for it in an email.
This topic comes up every so often. The anecdotal evidence from dog owners is that whilst any dog
can suffer from inherited diseases
more pedigree dogs are insured thereby distorting the figures. I couldn't say how true that is but it's certainly a plausible theory. Another commonly held belief is that pedigree owners are actually more likely to insure their pet than mongrel owners simply because they are aware of the diseases/conditions known in their chosen breed and feel their dog may be more at risk than other mixed breeds.
This is another reason why backyard breeders are so bad for the breed they choose to exploit. By knowingly (or through pure ignorance) mating poor specimens they can perpetuate chronic health problems that, due to necessary vet visits, will ultimately drive up the price of insurance premiums.
With regard to your proposed email - it depends on what you wish to achieve but, you're writing more as a knowledgable dog owner than employee so pitch it with your dog hat on.
Kindness is the language which the deaf can hear and the blind can see ~Mark Twain