Your response is my train of thought as well. I think some breeds are beyond repair when it comes to health, unless fresh genes are allowed to mix into the stock. Bully type dogs and some toy dogs come to mind. I have German Shepherd Dogs and some breeders can be quite passionate about choosing the healthiest lines they can. Even still, there are traits that seem more prone to German Shepherds and I am not sure we can blame all of that on “backyard breeders” and “oops litters”.
But I think we romanticize the mutt, thinking random mixing is better. I had one mix that I like to think that if she were allowed to breed would have added to the dog world. Another, although I loved her, had some traits that I would not have wanted passed on (although it might have been more of a nurture than nature issue). Without natural selection weeding out the weaker traits, I don’t see how random mixing of genes will make healthier and smarter dogs.
But I could be wrong. I hope this is one thing this study might shed some light on.