If you always limit yourself to purebred dogs, new breeds will never be developed. And yes, I think there is room for new breeds, because there are certain combinations of traits that do not exist in any current pure breeds of dog. (Obvious example: if you want a poodle-type non-shedding coat, and you want a purebred dog, you have very few choices about what other traits that dog can have.)
I agree with many of your other points (doodles are not a breed, doodles are not consistent, many existing doodles have problems, dogs for breeding should be carefully selected.) But I disagree about "purebreds only" being the solution.