Being infantry does not make you inferior or shitty
Swiss pikemen were the most elite force in the late medieval Europe (you know, that era on which Westeros militaries are based on) and those guys could not ride a horse when their life depended on it. Pikemen with archery support are a perfect counter to a cavalry, especially light cavalry with no armor
10 years old boy is about as far away from your average soldier as Gregor Clegane, Meribald hardly speaks for everyone. Again I have given the descriptions of the armies of the WOT5K, all of them are vastly different from Meribald and all of them mention trained and equipped men fighting in learned formations.
I shall rather believe the descriptions of armies we are given in the text by people like Tyrion over a random idiot who went into the war with kitchen spoon as a 10 years old
And again, Robert has not seen a Dothraki in his life. He knows nothing of them.
The real world is not in question, but the logic and common sense are. If you have no armor and your horse has no armor and you ride into a pike wall with volleys of arrows raining upon you, you are dead in every single world.