I think what you're missing with this statement is the rancher who has always used some other breed, one that is good enough for the tasks he needs done. If the dog gets the job done, and the rancher happens to like that breed, he has no motivation to go find something better.
Of course; I'm not in disagreement with this at all. Maybe my example wasn't clear enough; it was intended to illustrate a shepherd (or hunter, or whoever) had witnessed a Border Collie (or whatever hunting breed, etc) in action, efficiently and successfully doing the work they themselves needed done--better than the dog they had. Therefore, they gave one a try for themselves and found that the Border Collie (or whatever breed) did the job that needed doing better. As with my original post, I'm still referring to shepherds (etc) whose livelihoods or even literally their lives depend on their dogs (of any breed).
I can't imagine that someone depending on a dog to help put food on the table (directly or indirectly) could keep an okay-working breed just out of nostalgia.
On the other hand, if I misunderstood, and you or others simply disagree that this could be a contributing factor to breeds being lost due to the development of other breeds more talented at the job, that's okay too. I promise I won't try to make anyone agree with me by replying to each post with a 10 paragraph essay about why I think my point of view is right until I finally force people to just give up...