So you're saying there a two populations of people- those who re-home dogs and those who buy new ones. Lets suppose you're right. If i persuade 10 people today to move from the second population to the first that's 10 dogs lives saved. If i persuade 10 people to do the reverse, that's 10 financial transactions which have a net effect of encouraging the beneficient of those transactions to manufacture more product, and 10 failures to get people to save a dog's life. That's how it is being part of the problem.
Of course previous experience affects a dog's behaviour, just not in the way people usually think. And i did point out that my example was simplified as a demonstration, for the purposes of explaining how a dog's brain works. I have got 11 years of postgraduate training on the subject- i can elaborate if you like. The example i gave is not 'therapy'- it's part of a daily interaction with all dogs, all the time, that results in particular behaviours recurring or not recurring. This is true of any dog, wherever it comes from. And i repeat, just because a dog is rehomed does not mean it will have any problematic behavioural issues at all. If buying a puppy from a reputable breeder guarantees no behavioural problems for life, tell me why its the most commonly cited reason to request euthanasia in pedigree dogs?
You ask why should someone go to the trouble of reshaping the behaviour of a dog who does happen to have some problems? Because they care. Because its rewarding. Because its the right thing to do. Because it gives meaning and purpose to your life. Because nobody else will. Because the dog is not a commodity for our selfish convenience. Because they can. Because it demonstrates to others that its possible and they might try it. That's off the top of my head, i may think of more reasons later.. 