Can a dog live off a dog food with corn as the main ingredient? Yup. Soybean? Yup. These kinds of foods are fine for 90% of the dogs out there (aside from puppies, lactating females, and high end working dogs). Not an ideal way to feed, but no one has been able to come to a better overall solution yet and have it be affordable for everyone.
I have to disagree with this. A diet high in grains eventually catches up to a dog or cat. This is reflected in the increase in pet obesity, diabetes, liver disease (along the lines of NAFLD) and cancer. My friend's dog is a 9 year old sheltie and the poor thing is dying of lung cancer. LUNG CANCER??? I know Abby wasn't smoking unfiltered Camels and working in an asbestos mine in her spare time. The carbohydrate/cancer connection is very well established (even though the medical community is ignoring it). There are vets that have caught on to this and are warning their clients about the dangers of a long time diet of sugar (which is all that corn, wheat and soy really are once it is digested).
Sure there are some crappy veterinarians out there, but give veterinarians and veterinary students a little credit for trying to do what is best for your pet.
Vets should be held to a higher standard when it comes to animal nutrition. Unfortunately the nutrition portion of vet training is not nearly long enough and often sponsored by pet food manufacturers. If they are prescribing corn, wheat and soy for dogs, they are failing their patients and causing more harm in the long run. Prescription pets foods are very expensive, so for the same price are there are much better options.
Where on earth are you getting this information? I am a vet student and we just finished my 3 credit small animal nutrition course that was run by one of the top research nutritionist veterinarians in the country. I also have a masters that included nutrient digestibility of forages as part of my research. I am not saying you are right or wrong, but if you have the credible resources to back it up please send it.
I am sorry to hear about your friend's sheltie- but there is no scientific evidence that metastatic pulmonary neoplasms (lung cancer) is caused by eating corn, wheat, or soy. It is very common in boxers and bernese mountain dogs, occurs in animals around 10 years of age, and is caused by tumor emboli spreading by lymphatics and blood vessels. The reason dogs are suffering from all these problems you mentioned is 1) congenital defects of breeding pure breeds 2) owners not feeding animals properly (free feed/treats/people food) and 3) animals living much longer now than they ever used to before (wolves only live on average 6 years in the wild).