I wasn't aware about the problem with oily fish, usually for humans it is promoted as a healthy food, but as we all know, we can "go to work on an egg" and then we can have no more than 3 a week, so who knows because the guidelines we get change all the time! I can honestly say I have not seen ANY of the symptoms described in the link you posted thankfully.
I feed Burns and fish together usually, sometimes the odd feed just dry, and sometimes some of my own cats (two who are a little gingivitis looking in the gums - have been since being tiny) get a raw chicken wing (free range). I do not overfeed, so the overweight thing is not an issue. I am not happy to see grains in cat or dog food though and have considerer Orijen, but that is brought in from Canada and is not great for our air miles. I also add a little filtered water to the food when it is mixed with fish, so again the percentage of fish is reduced.
Mr Burns did indeed get into trouble claiming that the foods could fix this and that, but he had not registered them as a medicine type of diet, and in a way it is akin to face cream manufacturers telling us all our wrinkles will be gone if we use their cream.
The other day I boiled the bones for our Mum to be cat, and you should have seen the amount of jelly I managed to produce from the bones and the filtered water, it made me realise just how much the manufacturers can bulk out the food by adding such simple ingredients!
I have no idea what mammals die from mostly, that would be impossible to ascertain, as a lot of wild mammals are eaten by predators before they reach adulthood, and if they are sick die of their illnesses as they are not lucky enough to have veterinary treatment, I do know cancer is a major problem with 1 out of 3 humans dying from it, but I am not a statistician for animals or humans so cannot help you there. I try to feed the best I can afford, and have read and re-read food labels over many years, and at the end of the day the pet food industry is huge and is out there to make a profit first, and not worry about how healthy our animals are first and foremost unfortunately. But if we don't buy the poor quality ones then hopefully our animals will gain better health. I do not get paid by Burns or receive any of their food free for our charity or my own pets, and have constantly refused free hand outs from our charity because I do not want to feed those type of foods that may have been donated, and have paid the extra out of my own pocket, to give the animals a better start whilst they are fostered with me.
When the cats are at the vets the nurses often compliment me on how well behaved and happy the foster cats behave whilst in their care, even after having their neutering and spay operations, in fact it happened again today, I was told how friendly they were towards the staff, something must be working thankfully!