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Destriers are thought to be a cross between a light draft type and a athletic riding horse. Destriers were robust, muscular, very built. It wasn't some skinny fast Arabian. (It literally states in your reply that they differ in terms of muscular build.) A friesen horse or an Andalusion horse are thought to be good estimates of what Destrier was like. Both of these horses are massive compared to their counterparts. Just because they are the same height does not mean they are the same size.
Also if you look at Battle of Dyrrhachium, (1081) records state that Norman Cavalry charged the center of the Byzantine front and caused it to rout. Horses charge things in today's day and age. A trained warhorse would undoubtedly do the same. This is the middle ages were talking about most of the army was rabble. Aside from the mercenaries and heavily armored knights. It's not unlikely that horses charged into the best target. Which was nothing more than a feudal levy.
Training a horse to give risk its life up isn't impossible like you say it is. You can train an animal to do almost anything with the right tools and training.
The battle of Carrhae 53bc.
The battle of Patay 1429AD
And the battle of Kircholm 1605 are all examples of cavalry being used to break armies with devastating cavalry charges.
Kircholm is not an example, whatsoever. The PLC Cavalry routed the Swedish Cavalry, which retreated right through the middle of the Swedish "pike" blocks, which were advancing and assembling, and didn't actually have enough pikes to be reasonably called pike infantry, something Gustavus Adolfus rectified in his reforms on the heels of Kircholm. The Polish Hussars rode on the heels of the retreating Swedish cavalry, and as the swedish infantry opened their ranks to allow the fleeing swedish cavalry to ride off, the PLC cavalry rode right through those gaps. They didn't charge into the teeth of pike infantry. They rode through gaps made by enemy cavalry.
Any shock action undertaken throughout history by cavalry, has been the exception to the rule, not the rule itself. There are so many reasons why this is fact, but I'll just wade into a few reasons. First, cavalry, any sort of cavalry, that rides headlong into closer order infantry, with enough depth to absorb the initial crash, dies. All of it dies. Period. The first horses and men that go in, create a mass of dead and dying men and horses, that each subsequent rank of cavalry plows into, to meet the same fate, and create an even more impassible writhing mass of flesh.
Next, let's look at the social makeup of cavalry throughout antiquity and into the medieval period through to the 19th century. In the European tradition, it is almost entirely composed of upper class, nobility. Ramming the future leaders of your country/kingdom/state into a mass of peasants, isn't very smart. The value of cavalry, was in its existence just as much as its potential for carnage. It mirrors the fleet in being doctrine of late 19th early 20th century navies in a way. You needed cavalry to counter their cavalry, you couldn't risk it in a pointless suicidal frontal charge against peasants.
Let's address the economic aspect now. You can take 256 guys, give them 15 foot poles with a sharp end, spend a few weeks teaching them drill, and they are a fairly effective fighting force. It takes a lifetime to train a skilled cavalryman. The investment in resources to train a pikeman, or a crossbowman, compared to a cavalryman isn't a comparison. Cavalry is too valuable to throw into a meat grinder.
Why is it valuable? It's valuable for its presence, and the implications of its presence. As a rule, cavalry did not lower lances, and plow into close order infantry with sufficient depth to absorb the charge. It just didn't. There are exceptions of course, but as a rule, it just didn't happen. What it did, was charge, as in, the verb, to charge. This is an action, not an outcome. Cavalry charged, over and over again. It was playing a meta game of chicken with the infantry it was facing. If the infantry wavered, broke, loss order, the cavalry could, and would exploit. Plowing into a gap and riding through. However, if the infantry did not disorder itself, then the cavalry wheeled away at the last second, and reset, or it attacked obliquely in the cavalry form of a drive-by, lancing/spearing the corners of the infantry formation.
The true value of cavalry, wasn't in plowing into infantry headlong, that is either a ****up, or an act of desperation. The value of cavalry is psychological. Cavalry appearing behind you is a very bad sign. Even if the battle is going in your favor, enemy cavalry behind you, or on your flank uncontested puts infantry in a very precarious psychological state. Slaughter comes after the retreat in almost all cases, if you're confronted with cavalry uncontested in a threatening position, you're left with, ignoring it, or reconciling what it probably means, and that means you don't want to be the last SOB to turn and run, even when that is literally the last thing you want to do. This is how Alexander used his cavalry. Slip them into a place they shouldn't be, and watch the enemy collapse.
Economically, it makes no sense to ride cavalry into the face of ordered infantry. Socially it makes no sense to send the flower of your upper class into the face of a mob of peasants who can, and will kill them to the man, in a gruesome pileup of horse and man. Tactically it makes no sense, when the advantage of cavalry is to counter other cavalry, and to ride down easy targets when the actual fighting is over.
We do need to address a couple of other issues now. First, why we can't take all medieval historians for granted when it comes to the efficacy and battle winning prowess of cavalry. The vast majority of this historiography, was written by the nobility, the people riding the horses, and vested in giving themselves good PR. That said, there is a very stark contrast between different eras that cover the era of cavalry. During the early medieval period, immediately following the collapse of the WRE, this was undoubtedly the peak era of cavalry for Europe. We're talking about men who trained all their lives to fight on horseback, typically facing an enemy army that was primarily a rabble of peasants with zero drill, no real weapons or tactical training. This was the perfect environment for cavalry to operate in. Loose groups of ill-trained peasants with no discipline, or understanding of how to counter cavalry. Push a few hundred years into the future, and things change. The moment drilled infantry shows up again in Europe, cavalry is relegated to a support role for all the reasons I've gone over. They can't ride into a press of men, and not end up with broken horse legs, and impaled horses creating a wall of dead and dying flesh for followup rank to crash into. Since they couldn't do this, they certainly wouldn't waste their future leaders, or the tremendous investment to train these warriors in this manner.
Lastly, the vast majority of these examples of frontal cavalry charges, are not exactly frontal cavalry charges, ala Kircholm. There was something else going on that allowed it to happen. From a dragoon being shot dead and careening into a pike square, busting it open, allowing followup dragoons to ride into the square, and then pursuing the survivors into the middle of a second square that opened up to receive the survivors, to chasing fleeing cavalry into gaps they forced open in their own lines.