I disagree. Genetics and environment are equally important; a dog who is naturally predisposed to a certain behavioural trait but is never exposed to the environment in which it is expressed will never show the associated behaviours. You get just as many neurotic dogs from breeders as you do solid dogs from shelters. There may be more dogs surrendered to rescue because they have neuroses that the owners can’t cope with, but those neuroses aren’t created by the shelter. Conversely, you have many dogs who come out of a shelter, having spent years in less than ideal conditions, and have only relatively minor issues that can be resolved with a little commitment and training.
In the UK and USA, the majority of dogs are surrendered because of behavioural issues. They’re certainly not all abused or neglected dogs, simply dogs who were predisposed to certain traits through their genetics, which their environment triggered, that made them unsuitable for the home they landed in.
This is why ethical breeding is so important. With ethical breeding, where breeders keep temperament as high a priority as health, there would be far, far fewer surrenders in the first place.
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