You seem to have forgotten that armoured horses have received big nerfs in the last patches. Going for that Plated Charger just isn't worth it anymore. Lighter horses can catch up to you, outmanouvre you and kill you much easier than you can kill them. Or they will just couch your horse, instantly killing it. Versus men on foot, the horses are too slow and unmanouverable to get out of risky situations quickly.
Yes, I don't want every cavalryman to use the armoured ones. But I find it ridiculous that we have to pay so much for armoured horses while their stats are mediocre. For instance, yesterday I saw someone riding a charger. I haven't seen anyone riding that horse since months.
The repair bill on them is ridiculous at the moment.
Its manueverability is a bit of a drawback, but you pretty much covered it: other cavalry are the only things that can kill your horse regularly. Especially now. With throwing lances nerfed into oblivion, there is no way to consistently kill the armored horses if you are on foot.
Arrows: now cut damage. Against an armored horse, they barely scratch them
Throwing: jarrids, throwing lances, throwing spears, and throwing axes were overzealously nerfed, and not even a full stack of these can kill your horse.
Crossbows: The closest contender to the armored horse killer. Slow reloaded times and two-slots for crossbow alone make these an undesireable choice.
Pikes: Can stop a horse, but are ineffective at killing them.
2H: too risky
Polearms: Decent choice, but cavalry easily avoid these.
1H: cavalry bait
Whenever you have an item or a class where the only counter is the class in question itself, that class is too powerful. The real counter to these armored horse is the almighty dollar. People who can afford these horses are often the ones who decided which way the match is swung.