that is like saying "who is gonna pay for that kiddy porn as it's already been filmed may as well find a market to sell it to"
The market should not exist in the first place.
Your analogy is really a bit of a stretch.
Not all breeders are running "puppy mills". There is a market for purebred dogs, I see nothing wrong in this.
Really the problem isn't legit breeders, it is with dog owners who decide it's too much hassle and discard the pet by taking it to the pound or dumping it somewhere like unwanted garbage.
Incidentally, all of our dogs are neutered even the pet shop pup. She's a papered Rat Terrier.