What's the worst part of getting a puppy? Eventually it becomes a dog. No one has ever said "Man, this puppy is so annoying, I can't wait until it's a full grown dog." Would people buy more puppies if they were guaranteed to never turn into dogs? I say yes.
Here is my business plan for a company called Puppies Forever. You pay $1000 for a year long membership and we send you a puppy of your choosing. You keep the puppy for two months and then you send you puppy back to us and we send you another new puppy. So you end up having a puppy for a full year, rather than for a few month when just buying a puppy.
How do we afford so many puppies? Mutts. Nobody cares what breed a puppy is, as long as it is a puppy. Indeed, the only reason people obsess over certain breeds of dogs is to make the dog portion of owning a puppy more tolerable.
What do we do with the dogs once they are returned? Whatever, who cares. Exterminate them, train them to be seeing eye dogs, whatever.