I've read a lot of medieval military historians, they all say what I said above - cavalry did not charge head on into formed spear/pike/shield line. Not even heavy armoured cavalry. Maybe Polish hussars were an exception, but it sounds like they were equipped to strike effectively before reaching the line, not go into the line. Also, cavalry charges were at a trot, not at a gallop; which would be fatal as one fallen horse would cause a pile-up. They were a much more controlled affair and could be broken off if the enemy line didn't break before contact.
First, it is not lost to me that knights have some flexibility. They do not have to carry the charge through or attack at maximum speed, especially if enemies are not recoiling.
Second, knights were certainly equipped to attack spearmen. Their lances are about 4 metres long, less if they were poorly trained. This is good enough for most spears, just like hussar can attack pikemen. There was a tendency to make lance as long as possible, as advantages of a long lance are readily seen even if you are going knight vs knight. In later years however, lances have become shorter, probably reflecting decreased training.
Third, they were certainly trained to attack fully formed infantry formations. The knights usualy started the battle (if there were no missile troops) and only then infantry was deployed. In a way, their very purpose was to break formation.
Fourth, if we accepted that knights could not attack infantry formation we would have a real hard time explaining why cavalry dominated the medieval battlefield and why medieval strategists regarded infantry why such an obvious disdain.
I am curious what your historians can say about deployment of knights in battle. I am not however surprised by your statement; realistic estimation of knight effectiveness is rare, especially in books that are not themselves dedicated to knights. Many books mention that medieval tactics was rigid and inflexible, but fail to say why this was so.
Of course, some allowances must be made for differences in knight effectiveness. Knights dominated in Europe for almost 500 years and would differ somewhat by country and period. There is also more room for training being of importance, more so than with infantry.