And 100,000 against 100,000 ?

Wouldn't you underestimate the carthagian cavalry at Cannae for example ?
I agree with GueulEclator. Untill the perfection of fire arms a good cavalry has always been the most powerful weapon in open field, due to it's maneuvrability, speed and strenght. The addition of a horse, as you like to call it, adds power to the rider's weapon, adds to his view of the situation, adds to his possibility to choose his ennemy, it adds also to the fear he causes to the tiny infantryman... of course, there are effective ways to fight the cavalry and to defeat it, but you need terrain, you need tactics, you need experience, you need luck and you need the appropriate weapons/equipment.
And we are in the Middle-earth, aren't we... the losses the riders of Rohan or knight of Dol Amorth suffer at every engagement with the most average ennemy orcs or uruks in the game are simply not correct. In my game I have risen the defensive stats of all cavalry, and it made it more effective, and they don't get slaughtered as in the vanilla game. Sure... they can't charge a spearwall, but once they charge into the ennemy flanks or rear and the melee follows, they are much more resistant. A broken infantry formation in open field cannot, with possible exceptions of very experienced heavy or special troops, withstand the combat with riders.