OH MY is this soooo untrue, look to ARPH/STAAR (aussie rescue) and Border Collie rescue and you WILL find well bred dogs in shelters, dogs that have excellent lines behind them but were sold in irresponsible owners who did have the time nor desire to admit they were wrong in buying the dog and simply dump.
No, its true. You will find many many more poorly bred dogs in shelters than conformation bred dogs. I work in a shelter and have yet to see a conformation bred dog there. I'm not saying it never happens, but these dogs don't line shelters at all. We get purebreds all over the place but not well bred purebreds.
Even if a well bred dog manages to find its way in at least in my breed they get out really fast. Any breeder who fins out one of their dogs ended up there will have it out asap.