Horses are different from humans. Horses don’t know there is anything coming up to work at, so they personally are not ready for races, the humans only try to make them so. And no one wants to hurt them, and cannot know really what is going on with them that might hurt them including “just not feeling perfect”. A human knows if he just cannot go on and can bow out, but he also knows if he wants to push himself to a set schedule that is required to get to regionals, nationals, etc. One just cannot do that with an animal.
Thus, there will never be requirements that horses go from one thing to the next. It is unfortunately one of the impediments to racing being popular these days.
The Triple Crown is a simple construct. Each race is its own entity and of its own importance. We artificially put importance on this “TC” being a string of events that just happened to become very important IN THEMSELVES. And that is why the TC exists at all - because people thought all of them were of high importance. It really did not exist until the ‘40s.
That maybe why I prefer the Breeder’s Cup.
My family spent many years and many dollars rehabbing racehorses for the show ring. I've seen crushed navicular bones, broken sesamoids, splints, nerved legs, and other horrors horses have endured and kept going. Thoroughbreds will keep going until they literally break apart. It's the responsibility of the owners and trainers to know when they've reached the limit. And oftentimes, owners and trainers will be at odds.
In my opinion, the way they have the TC set up is detrimental. People will push the winner through the races because they would be risking too much money, time and fame not to. The TC should be run in May, June and July. That would take the whole spoiler element out of it and put all the horses on equal footing with regards to recovery.
And Sir Barton won the first triple crown in 1919.