As an avid animal lover and as someone who works with animals, this is my view:
Only purebred dog I ever had was our collie, who had "collie eye syndrome" which is basically tunnel vision with poor eyesight in the front. She's pretty much like a third blind. Almost every purebred I've known has had breathing problems, or eye problems, or hip problems, etc. It's not breeding persay that's the problem, but that we've OVERBRED dogs to the point where it's basically inbreeding.
All the breeding for very specific desired traits has allowed the bad ones to flourish as well. It's the same thing as if you bred your cousin, then your kid bred with their cousin, then their kid, and so on. Eventually do to the lack of diversity, mutations and other problems will become more common.
This is exactly why mutts are my favorite breed. Because they have more genetic diversity, they often live longer, have less problems, are less prone to chronic conditions like hip displaysia, etc.