I personally wouldn't recommend Iams or Pedigree. You can find some higher quality foods for the same price roughly and a good food can really do wonders for a dogs health, energy, coat and more.!
You know, I hesitated to chime in because I know many people believe this very strongly. But then I thought, better to have the conversation and see what we all can learn.
I have no doubt that there's a lot of truth to what you say. But here is what I know: Marley was fed IAMS by his previous owner and we were trying to keep things the same as much as possible. He is now 4. He's never sick. His coat is beautifully soft. He has a ton of energy -- we do our best to tire him out, but never quite manage. Most of the other dogs he plays with tire out long before he does. He is quite "regular" and his poops are firm and normal looking. And plenty of dogs live to ripe old ages eating these sorts of foods.
So, what's the answer here? Maybe it's just that a strong dog, like a strong person, can get away with eating things that others can't. *shrug* But my thought is that if something isn't broken, I won't try to fix it. There are too many other things to worry about.
And the reason we didn't try to regulate Marley's diet is because we didn't want to make him totally food focused. I see a lot of dogs like that. Of course, if the dog won't self-regulate, you have no choice.
I respect other people's food choices, and I am always interested to hear what people are feeding their dogs. Things can always change.