All the same uninformed arguments show breeders use. Breeding for the show ring is what and to coin a phrase "fucked up" the Doberman not backyard breeders. A doberman is already a mix of breeds thats how it was created, like all breeds of dogs. They didn't just drop out of the sky fully formed, they're not a species they are type of a single species the grey wolf all dogs are. Thats also how it will be saved, if anyone truly cared about the Doberman that is and not instead only cared about producing a litter of fatally inbred mutts to win ribbons in the show ring.

You're mixing up environment and genetics. Doesn't matter if some owners or "pet dog" owners never worked their dogs if they were bred along working lines by anyone instead of showing lines they would be quite capable of working, generation after generation doesn't matter who kept some of them or how. However this isn't the case with the Doberman. Breeding for the show ring means they're selected for a showing phenotype, size, exact proportions, arbitrary things like depth of stop, eye shape and exaggerated movement......not for how well they can perform, jump  drive, eye contact blah blah blah all the things a working dog needs to be selected for. By doing this they are creating an image of a dog not a working dog. To make matters worse they inbreed to winning sires on mass hoping to produce a similar champion, they line breed to that single champion litter after litter, many breeders all reducing the gene pool. Until all they end up producing are crippled genetically inbred disasters that live just the few years it takes to become champions. What we call a Doberman today. 

Outcrossing both to other breeds at this point and within the breed if even possible is vital even if there is a hope just to save the show dog never mind a working dog. You have a point in that the Doberman seems to have fallen out of favour as a working dog in favour of any number of other breeds. Show breeders will often take such a breed and believe they're rescuing them by breeding them for the show ring but instead wreck them. If I was to choose extinction or  bred along show lines I would say extinction is preferable. Kinder at any rate.

Backyard breeders don't come into it at all. In fact you are more likely to find a healthy dog from a backyard breeder simply because they don't agonise so much over pedigree and dog shows so are just as likely to use a complete outcross on their litter as they are to inbreed, a simple query will tell you which. However show bred dogs carry the fatal genes, they created them and they persisted in breeding with them and not addressing the problem by outcrossing. Any cast-offs deemed "pet" quality will carry the genes too. Bear in mind any dog can be registered with the AKC if it has a pedigree, backyard breeder dogs, puppy mills and show dogs. So yes the blame lies fair and squarely with show breeders here.

Your argument for not outcrossing is like saying I don't want to build my house out of bricks because I don't have exactly the ten thousand bricks needed, if I use one stone instead of that missing brick its not a house is it???!!! Well it is a house and that one stone might be the only thing that holds the whole together. You would rather build a wooden house instead that gets flattened by a tornado. 

That was a rather obscure off the cuff metaphor but think of the doberman as the house and stone as the outcross. In three generations you wouldn't even know a stone was there in the first place and the dog would've been saved. Of course it might simply be too late because there are no bricks left after all that procrastination, no healthy Dobermans left to backcross to. Which might actually be the case anyway.