What you are missing and everyone else is that the AS reduction niche for armor items is already occupied by both Frozen Heart and Randuin's Omen. BOTH of those items hinder your enemy's attack speed and both are high in armor value.
Thornmail, by contrast, exists as an item that supports you for building heavily tanky, but also punishes opponents stupid enough to attack you. The niche for this item it to be the tank that is coming to bash your face in, but you don't want to attack because it will hurt you. The niche of this item is to counter high AS champions, who often favor AS over raw damage, by dealing a lot more damage to them for attacking you.
By outright slowing down their attack speed by the Cold Steel passive, this muddies Thornmail's niche entirely, and shoves it into the niche of Randuin and Frozen Heart. This is not something that needs to be done at all. I could understand this passive if the reflect damage was increased to compensate the drop in enemy AS - since the damage was decreased, though, Thornmail will sting a whole lot less than it does in live, which will leave it in just as bad of a spot as it is now.
Thornmail needs to have it's niche amplified, not shifted into a niche that's already occupied.