You have a lot of assumptions in you. Most kill shelters I have been to only put down sick, old, or too agressive of animals. I have been to others that have time limits for killing them as well, but not all of them are the auschwitz that you are making them out to be.
Also, you are saying that any dog that goes to a shelter is not worth it to save? How twisted is that?
Also, buying from pet stores that buy from puppy mills creates even more dogs out there to possibly go homeless.