Couple things,
There are two types of white.Recessive white, which takes two copies to show itself, is much better at covering all color. This is the white of white Cornish, at least the ones we raised.
Dominant white, which will show itself with only one copy, and covers black pretty well, not as effective with gold. This is the white of white laced red Cornish.It's only the white of white laced red that is dominant, although their may be red enhancers involved that may be causing some bleedthrough.
I don't think that Cornish breeders are not interested in expanding the breed, as much as they are uber cautious in who they are willing to trust to carry on their work, and precious blood lines. They get hit up every day by people who want to cross them to a rock or other breed to make their own broilers. There by pissing away a lifes work with these birds.Even if newbies interest is to keep and breed pure Cornish, the breeder is skeptical if they will have the dedication to stay with such a difficult and challanging breed. They have seen it before, after a few years newby decides it's way easier to raise ( insert breed here) and give up on the Cornish, again resulting in loss of precious rare breeding stock.
What may be interpreted as indifference on the breeders part should maybe be looked at as part of prescreening process to weed out the casually interested.