I highly disagree.
The fact that most aquatic fish are so colorful is a direct result of human intervention. You don't get Koi Angelfish, Cobra Guppies, Electric Blue German Rams and the like by just breeding a couple unrelated fish.
It happens over many generations of (often) fish that are siblings. Without the introduction of new blood (which reduces profitability as you will not get your fish to breed true), your fish will gradually become genetically inferior. Breeders (typically) breed for color, size and fin shape. They do not breed fish simply to create a hardier strain.
To deny the effect of inbreeding on a fishes health, resiliency and vigor is a sign of an inexperienced aquarist.