My grandfather's best friend, Mr. Sherman, is 96 and has been smoking a pack a day of unfiltered Pall Malls for over 70 years.
Using the 'I know so-and-so who feeds Purina' arguement, we should all smoke Pall Malls to ensure living to 96, right? No. Of course not.
There are tons of people who have pets who are fed crap food every day of their lives and live to 18 years of age. There's a good reason for this, if the animal was going to be killed by crap food it would have been dead LONG before.
My cat, Mordred, died of acute liver failure at age 2 1/2. He was fed nothing but high grade food from the day I adopted him. He would still be here if I had just fed him Meow Mix, right? Hardly.