Interesting question that probably only the real "artists" can answer.
I used to think that way too. Until I experienced several outcomes of breedings that led me to start this thread. Again, IMHO, I think that even if you have mediocre koi, your chances of reproducing good koi is almost nill. You may get other mediocre koi, but my guess is that a majority will be junk (I'm just guessing) However, I'm sure mother nature may throw a curve ball into the spawnings every once in a while, but can you find that needle in the hay-stack? I would guess if you don't know what you're looking for, probably not.
Who knows, some hobbiest may end up with a winning breeding pair that cost $1000; that threw excellent koi offspring. What a great dream come true. You would also have a lot of jealous hobbiest...hahaha. Why do you think the "artists" use and keep "the best" koi that they have for breeding to strengthen their lines. I beleive they want to give themselves the best chances for success.
Locally, speaking to some of the club members in Hawaii who breed regularly, they admit that it is really difficult to produce koi that they would want to keep for themselves. And these breeders use expensive koi with proven bloodlines. Again, it is also about matching breeding pairs...Yet another topic that will take 10 years of breeding experience to even comment on. These long time breeders work hard at strengthening their lines, and still they often are disappointed with their outcomes. Even Grand Champion Koi, often are retired because breeding wise, they do not pan out...Just my two-cents...again!