Interesting points, but I don’t think that winning times are a particularly useful metric in horse racing. The Epsom Derby has seen its winning times stay basically steady since the 1900s. There are plenty of reasons for this, most obvious being that the horses don’t stay in lanes and there is plenty of jockeying for position. Indeed, the winner often comes from the back or middle of the field at around the half-way mark. In short, you need a good jockey and a horse with an impressive burst of speed, not an incredibly fast horse with staying power – an equine equivalent of Michael Johnson. Those qualities don’t show up in winning times.
What would be interesting, however, is to know the last winner of the Epsom or Kentucky Derby that wasn’t a thoroughbred. When thoroughbreds stop winning and the Emirs stop paying large amounts for them and their stud services, I’ll be convinced that selective breeding has stopped working.
And I suspect the purest bred dog is in fact a wolf.