[miranda_rights]

You're using the ad infinitum fallacy. At no point did anyone mention that no one should not use professional breeders. The point is, "most people don't know how to find a professional breeder". It's exceedingly easy to let 2 dogs of the same breed have puppies. It's a lot harder to screen for personality and physical issues. Additionally, there will always be people buying from professional breeders. So many fields need working dogs for protection, scentwork, hunting, servicework, farmwork, and I'm sure many other fields I'm missing. Additionally, there will always be a demand for show and sport dogs (agility, herding trials, schutzhund, dock jumping). This argument of "let's suppose there is never a purebred dog again" is very confusing for that reason.

Additionally, you would be shocked at the number of people who buy a cute puppy and then are surprised because they didn't realize their adolescent goldendoodle needs more exercise then a 20 minute walk everyday, or that their once friendly cattledog puppy is grown up and barking and lunging at every dog that walks by. Again, that's when the onus is on good breeders to ensure that the new owners know what they're getting into. Usually the facebook/craigslist breeders are not good about explaining or requiring this - they'll give away a puppy to anyone who pays. If those same people go through a rescue, they're usually informed by the rescue the amount of work involved, and can do things like foster-to-adopt to ensure the dog fits their lifestyle.

I love good breeders! They're absolutely fantastic about keeping their puppies healthy, and frequently have clauses about how the dogs must go back to them if the new owners want to give them up. They have careful screening processes and are good about making sure the owners will do the requisite work and training for the dog to be well-behaved and for everyone to be happy. Bad breeders do none of these things, and make the likelihood of genetic disorders higher, not lower. These breeders don't get their puppies checked for the 'bone issues that can be fixed at birth' as you say, and let the unaware owners deal with the fallout.

I'm also confused about your point about irresponsible shelters. Where do you think the animals would be if the shelters didn't take them? They'd be running around on the street, unvaccinated and untrained. Do you really think that's better?