I would be somewhat surprised if he isn't retired after the Belmont.

Just look at those muscles.  That's not a Thoroughbred.  It's a Quarter Horse.  Thoroughbreds in general have even skinnier legs and ankles than Quarter Horses.  Thoroughbreds are in general the fastest riding animals only between the distances of roughly half a mile and two miles.  Over shorter distances Quarter Horses are faster, and over longer distances Arabians are faster.

Second by 4 3/4 lengths is really a better performance, given that Eight Belles was a filly.

Legs move quickly, and weight in the legs is worth several times as much elsewhere on the horse.  All else being equal, the horse with the smallest leg bones will win.  People are going to breed the horses who can win.  The only way out I can see is to assess a weight penalty against horses with leg bones which are too small.  This is really not so much of a problem yet, at 0.2% of starting horses dying in the US, but that's up from 0.05% a few decades ago and it's only going to get worse.  It's about 0.13% at Aquaduct, about 0.35% at the major track in California, and about .03% in Hong Kong.  That sounds low, but if you have 100 horses racing over a day, that's about a 20% chance of one of the horses dying on any given race day.

There's also the problem of lung bleeding.  Horses have always bled a bit in their lungs when run hard, but it's getting worse and worse with Thoroughbreds.

There's also the problem of the closed stud book.  Let's for example say I take a champion Arabian, breed it for three generations with Thoroughbreds and due to hybrid vigor get a horse which can break the world record and run a mile and a half in 2:20.  Can I race it?  Nope.  It's not a Thoroughbred.  The big races are for Thoroughbreds only.  Can I breed it?  Nope.  It's not a Thoroughbred.  It's only 7/8ths Thoroughbred.  Not good enough.

Once you close the stud book and stop accepting outside blood, whatever your breed it starts deteriorating due to genetic drift, to a degree which is roughly the sum over all individuals ever of the cube of the fraction of the blood from that individual in the current generation.  If you look at Thoroughbred bloodlines, they're full of the same hundred or so champion horses, which is just barely O.K. if you have the same amount of blood from each of the hundred but is certainly in the danger area for overbreeding if you aren't distributing the blood from the champions evenly.

I remember when we took the Samoyed I grew up with in to get bred.  We only did this once, since she started biting people around the puppies.  Anyhow, she had extra long hair even for a Samoyed, which is the trait they were interested in.  They didn't like the pink spots on her nose.  OK, it's be a little better if her nose were all black, no problem.  But what they really didn't like was that she liked to spread her legs and pull, instead of prancing.  Hello?  Isn't this supposed to be a sled dog?  Aren't they supposed to be sturdy and not be fragile and prance?

Anyhow, the professional dog breeders will tell you that what we need to do is leave all the dog breeding to them, so that we can breed only a few the best animals, and get a high percentage of genetic material from show champions.  They say you should stay away from puppy mills because if you let dogs breed randomly you get dogs with shoulder problems.  Yet the reason the random breeding gives dogs with shoulder problems is the genetic drift caused by the reputable breeders.  It's the reputable breeders who are making the genes worse, and the puppy mills which are cleaning it up by letting the genes for bad shoulders be expressed and removed from the population.

You could ban the drugs. Doing so would put many small time trainers out of business, and probably result in some of the smaller tracks closing. But you would probably have that consolidation in the long run anyway.

I don't see how you get past the incentives stemming from the Derby. The biggest races in Europe are turf races often at longer distances and elegible for older horses, only durable horses will be the winners of those kinds of races. Not only is there a lack of durability, the Derby has skewed things so much in America most Derby winners don't truly have classic distance stamina.

Full disclosure: I know nothing about horses other than their poop is good for gardens and they can kick and bite if you're not careful.

That being said I wonder if you could create a metric for robustness...sort of weight per inch of ankle circumference.

WWGE: Who Would Gordon Enslave?