Thornmail is useless.

I faced a mostly ad team with only a nidalee as the main source of ap. I went full armor malphite (now don't bother mentioning how i did its irrelevant) What i had a problem with is that Master Yi was capable of 5 shoting me constantly with just 5 auto attacks. With a Ninja tabi and thornmail i should be impossible to deal with (I had 75 percent AD damage reduction from my armor) Is this really balanced? Why bother having thornmail in the game if they take no damage from the item? Malphite is the highest armor champion in the game and yi only had 1 lifesteal item (botrk) Does phantom dancer reduce the damage from thornmail? If it does i dont understand why bother having thornmail in the game. It only scales with 15 percent of the damage delt! This use to be one of my favorite items when i played rammus, but its useless even on him.