The problem with this is that, unlike the eggs of caged hens on the supermarket shelf for example, the puppies you see in the pet shop window today are living breathing creatures who don't go away just because they aren't bought by anyone. The longer they stay in that pet shop window, the more likely they are to end up with problems due to their vanishing window for socialisation, not to mention the fact that living in a pet shop window is not a particularly pleasant experience in the first place.
Someone falls in love with that puppy in the pet shop window. But they have been convinced it's wrong to buy a puppy from a pet shop, as they would be supporting puppy farms that way. So they don't. Seven months later, they go to the RSPCA to do the right thing. They adopt a puppy, around nine months old, who is fearful of children and cars, and not house trained. They put in a lot of work and money to help this pup become comfortable in the world. They have done the right thing - but it's the same puppy.
It is our moral obligation to reduce the suffering of animals in our power. Whether purebred or crossbred, dogs should be bred in a way that ensures their well being. But let's not make the innocent victims of the existing system, the puppies who are here now, suffer to achieve this end.