But if everyone did that, distinct breeds would no longer exist

edit: Here come the downvotes, so I'll elaborate. I think the vast majority of people should get their dog from a shelter. However, I also think breeders have a place in the world. Left on their own, dogs will mate more or less randomly. While that would reduce the rate of genetic diseases through hybrid vigor, it would also greatly reduce the variation in behavior and appearance that defines each breed. Throwing away a couple thousand years of selective breeding seems like such a waste since the variation between breeds is a big part of why people are fascinated with dogs.

On the other hand, puppy mills (and the shopping mall pet stores who buy from them) are the work of the devil. The dogs are bred specifically for mass consumption and most of them are purebred due to the higher price people will pay for a purebred dog. Unlike a good breeder, little or no consideration is given to eliminating congenital disease from the gene pool. The impact of independent breeders on shelter dog adoption is a drop in the bucket compared to the impact of puppy mills. In my opinion, puppy mills have no place in the world and should be the primary target of animal rights advocates.