I have zero abiltity to judge whether genetic concentration is getting to the point where it is bad for the breed and therefore bad for the business long term. If it IS bad, my gut says you are never going to get individual players in the industry to voluntarily agree to use lesser sires for the general future good of the sport, so you 1) might need some top down organization in a sport that doesn't have clear leadership. Either that or 2) have breeders with so much money that they can afford to care for the good of the sport over immediate profits. This is horse racing and this is America, so I'll guess it's 2).
I don't like solutions where you tell someone what they can or can't do with their money/property, in this case a successful stallion who might be gone from an accident or disease at any time and so who knows how many books you can get. I actually think the way to solve the problem is to change the incentives to make RACING more profitable than BREEDING, and to have a broad pool of owners who put their money in training healthy and durable horses to win races, rather than competing for a limited or unlimited number of breeding lottery tickets. Of course for racing to be profitable you need larger wagering pools, which requires marketing and organization and leadership, which brings us back to 1) above.
1. Mr Hughes of Spendthrift actually said THIS with a straight face. "We are really concerned about the small breeder's ability to survive this.”
2. This is what The Jockey Club is worried about.
“On the mare side, in 2007, 5,894 mares (9.5% of the total) were bred by stallions that covered more than 140 mares. By 2019, 7,415 mares (27% of the total) were covered by stallions with books of more than 140, a threefold increase. “The combination of these changes has resulted in a substantial increase in the percentage of foals produced by a discreet segment of stallions—signaling a worrisome concentration of the gene pool,” the press release stated.