Perhaps human performance will plateau within the next decade. It may be nobody will ever run 100m in under 9.70 seconds, even with a good sire and dam. I wouldn't worry too much about the thoroughbred industry as the world will have a lot more pressing issues within coming years. I predict this will reduce both the gambling dollar and increase the costs of breeding and training. It is interesting to note that some of the worlds fastest horses are by unfashionable Australian sires eg Takeover Target (Celtic Swing), Silent Witness (El Moxie) and Absolute Champion (Marauding). As yet we don't give local Melbourne Cup runners asthma inhaler medication so our stamina lines don't show up so well.

The success of unfashionably bred sprinters suggests random variation rather than bloodlines may be important. I note that the Danish royal family is outcrossing with some local stock.

Edie Sedgwick

Tom Quirk, the answer is simple.Humans have not used running as an evolutionary advantage to escape their enemies or catch their prey,where as horses have pushed the boundaries of their physical mechanics over millions of years for speed and thus have nearly reached their evolutionary potential.Humans on the other hand have relied on intelligence rather than speed to out wit their quarry,hence that have not found it necessary to push the boundaries of their physical potential.

Horses are presently more than twice as fast as our fastest olympians and it will take millions of years of evolution for us to catch up.

I am not sure tha answer is simple, as suggested by Arjay. Very little is simple. However I am reliably informed that in the sport of pacing race times have reduced substantially as horses are bred to use this unnatural gait. This would seem to support Arjay's suggestion that in galloping and trotting (both natural) at least horses have come very close to their evolutionary peak.

Humans have the psychological factor. Some of us will remember when it seemed like no-one would run a mile in under 4 minutes. But after Roger Bannister proved that it could be done, all of a sudden it seemed as if if everyone was doing it.

http://en.wikipedia.org/wiki/Four-minute_mile

Similarly, Everest seemed unconquerable until Edmund Hillary and Sardar Tenzing Norgay successfully climbed it. Next thing we know, all sorts of people were up there.

Horses, and other animals, just don't have the psychological advantage of "well, if he/she can do it, then I'm damned sure I can!".