It's kind of interesting that while small breeds normally outlive larger breeds, the 3 oldest dogs on record are medium sized working dogs. I need more information about them because this is interesting.
I wonder if they were sterilised and if so, at what age?
I also find this interesting - my suspicion is that it's because structurally, the medium sized breeds are the closest to their wild ancestors. Their skeleton and internal organs haven't been squished into a tiny package or stretched out into a giant breed. There are some chi's having problems with their brains because of their dome shaped skull and being bred to be tinier and tinier, while a typically sized dog brain tries to grow inside a deformed skull.
Off-topic again, but would like to hear your thoughts on steralisation in pet dogs... I did a lot of research when I got my girl and decided to go for a delayed spay. Waited until she was two and fully mature before spaying, since I wanted to give her the time to mature physically and mentally, with all the hormones involved, before neutering. She never came close to being mated since I kept her close when she was in season.
Let me make this clear. If meat was the healthiest thing I would totally feed it to my pet. But if there is a better solution and the dog won't suffer, o will feed it the better solution.
But you still haven't shown how it's a better solution. You said that you don't just take things on authority, but look at the evidence before deciding. You've said that it's possible for them to survive on it - and no one is arguing that it isn't! But you haven't demonstrated that it's better, or that it's the right and natural thing for dogs. You've even agreed that dogs have evolved to live on meat. But you don't think they should eat meat anymore... why?
Be honest - both with yourself and here... Is this decision based on your faith rather than reason? Since your faith calls for a vegetarian diet. You swear that that isn't what's leading your beliefs here?