That seems backwards. Why would any ruling class want to put itself on the battlefield if it could avoid it? I'm aware of the warrior culture that grew up around knighthood, but cultural ideals that run up against practical necessity tend to lose out over the long term. When crossbows became popular as an answer to armored knights, the Church pronounced anathema on their use against Christian foes; that didn't stop people from using crossbows, though, because they worked.
If a large force of lower-class infantry is more effective and economical than a small force of aristocratic cavalry, then any kingdom relying on the latter is at a double disadvantage. Not only is their army inferior militarily, but they've also got their leaders out in the field getting shot full of arrows and hacked up with swords, while the enemy leaders are sitting safe in camp plotting strategy. It's hard for me to believe that in a fragmented and warlike society such as feudal Europe, centuries would pass without anybody figuring this out.
If the aristocracy is doing the fighting, it's because the state of war-making technology demands expensive weapons and armor... or because the state of agricultural technology is so poor that only the aristocracy can afford to take time away from subsistence.
I find it much more credible that the political and social power of the knighthood arose from their military value than the other way around.