With all due respect Dick. I think this is an old wives tale. I would like to see some concrete evidence to that effect.
Known and proven oya give you a higher percentage of usable offspring. It's a numbers game. But you are still going to throw away at least 99 offspring out of 100.
A less-discussed advantage of known oya and known genetic lines is the predictability of the offspring. When you are working with the same individuals (or at least the same bloodline) year after year, you learn how to recognize a good offspring at a younger age. You are better able to predict their development. This has some dramatic effects on the bottom line. You can keep fewer fish during the summer cull and end up with just as many good tosai in the spring. You have to handle fewer fish during the second and third summer cull so the labor demand per tategoi declines. Your keep-fish are growing faster because they have more space, better water, more food, etc.
-steve