I really hate the whole "all breeds started out as mixes" argument. It is absolutely not applicable to the designer breed argument, because the process is totally different. Labradoodles that are bred are most often gen 1 Poodle x Lab mixes rather than breeding Labradoodles to Labradoodles. How is this an attempt to start a "breed"? They did not develop any sort of a plan as to what the breed would be (is it a companion breed? A working breed? A hunting breed? How does one "prove" a Labradoodle?) , they took the easy way out and just decided to mix poodles and labs for the purpose of having a lab temperament in a hypoallergenic body- which 50% of them don't even have. Genetics don't just work that way, you can't just cherry pick which traits are passed down from the parents- which is why purebred breeding is so much more consistent (if done corectly). This also applies to sport mixes, who are bred to be successful at sports. To me, being "good" at flyball is a pretty complex thing- you need stamina, speed, quick turns, retrieving skills, control, biddability, solid jump skills, and balance... So while it may seem like sport crosses fill that niche, I have to wonder how may sport mixes don't make it. What if you got a BC rear with a more upright Whippet shoulder assembly? Or what if you got the topline of a Staffy and the angles of a Border Collie? My problem with mixing these breeds like it's no big deal is that these breeds were developed for entirely different purposes- and therefore have incredibly different structure and temperaments. What happens to the dogs that get an unfortunate mix of traits (I.E. Dog aggression inherited from the Staffy side, reactivity/shyness inherited from the BC side)? If a sport mix is not cut out for sports, can it be a normal, well adjusted member of society?
The purebred dogs we have today were developed by mixing breeds, yes, but they did it over years and years, with a clear goal in mind, and they achieved that goal by bringing in multiple breeds and really being consistent and strict with their breeding decisions. An example of that in current times is the Silken Windhound- which is already showing a lot of consistency due to the determination of people within the breed.