i have never understood this logic
if nobody adopts a shelter dog then yes it does die
how is that not true
Because it assumes that the shelter dog and the dog purchased from a breeder are the same, with equivalent traits. This is only very rarely the case. Most often, they are apples and oranges.
Now if someone is just looking for a sound family pet and doesn't have particular requirements for breed, size, color, etc... then certainly they may be able to find this dog in a shelter. If they are looking for a sound family pet that is also a purebred of a specific breed, their chances drop but it is still possible.
If, as has been said previously in the thread, someone is looking for a dog for a specific purpose... competition in any number of sports, police work, SAR, guide work, service work, show, then the chances of them finding such a dog in a shelter are very slim. If they want documented pedigree and known ancestry on the dog... which isn't just for ego but can provide valuable insight into any health or temperament problems that may be present in the bloodlines and thus arise in the future... then the chances of finding that in a shelter are pretty much zero.
So bottom line, for many pet owners the shelter is a viable option. For pet owners looking for something specific, it often is not. For people looking for dogs for sport or work, they'd have a better chance finding a needle in a haystack in most cases. Thus, there is really no competition. The shelter doesn't offer what some people want, and breeders are the only option. Likewise, the breeders with proven track records of producing quality dogs that can succeed in a variety of endeavors are typically not the ones whose dogs end up in the shelter.
So it is absolutely NOT correct to say that when they buy from a breeder a shelter dog dies, because what they want isn't going to be found in the shelter.