We could probably learn something about sound, healthy horses, with less tendency to break down, from other countries, who to my understanding, have a significantly lower equine mortality rate than we do. Continents like Europe also run predominately, or exclusively, on grass, which is probably much kinder to horses than dirt. As far as our horses from yesteryear and the distant past breaking down, that's probably true, but we also run predominately on dirt over here and the bulk of our racing consists of lower level claiming races where we grind these poor horses out day after day. I don't know what kind of levels are run in other countries, but I suspect the claiming ranks are not as dominant outside of the U.S., and the racing schedules are mapped out differently, although I don't know the specifics of these. There were super horses in the old days who would race a lot, but I also suspect they trained less or trained differently (just a guess, I really don't know). But the majority of horses were not super horses, and I even suspect (I'm speculating) that many of the big stars who raced so much or with high imposts, saw a toll taken on their bodies, depending on the circumstances. These horses didn't break down, but too much racing, or too much weight, can take a toll on any horse, and if the horse is fatigued or his soundness is in any way compromised, he is more predisposed to injury. When Man o'War carried 138 pounds, as a three-year-old, in the Potomac Handicap, he set a new track record; and he also compromised himself with a bruised tendon. There is no horse, from the lowest level claimer to the greatest horse who ever lived, who is not vulnerable. These are fragile creatures made of flesh, blood, and bone. Man o'War wasn't a machine. Citation, who was overraced, IMO, wasn't a machine, and nor are any of these horses running today, none of whom are even in the same racing universe as these two.
I know that the synthetic tracks are attempting to address safety issues in the U.S., and are welcome despite the grumbling from certain people. While the synthetic tracks have been shown to reduce fatal injuries, there seems to be an assortment of different kinds of injuries on them. At any rate, perhaps the ideal would be well maintained turf tracks, but to be fair toward the synthetics, these surfaces are still very recent and only a more protracted period will determine if the synthetics are indeed the best. As we go along, the tracks and the studies gleamed from them, should result in "fine tuning" of these surfaces, constantly improving the product, so perhaps the synthetic track of tomorrow will be different in terms of its quality, safety, and durability than the ones of today. The ongoing injury reporting system that was launched within the past few years has been long, long overdue. With regard to the synthetics again, the Europeans, who race on grass, seem to really take to them, and this is an extremely good endorsement.
The link below is to a good article by Heather Smith Thomas in the September issue of "California Thoroughbred". It doesn't discuss breakdowns in general but rather on one specific topic, as it relates to different kinds of surfaces: shoeing. It is very good reading.
http://www.ctba.com/09magazine/sep/HorseCare.pdf