Thornmail change

Thornmail is a good item, but i feel that it hurts the best champ who buys it "Rammus". Lowering attack speed of someone taunted is Constantly the difference between a kill and a flash away carry. So my thought was thornmail should (lower attack damage for 2 seconds instead of lower attack speed) the actual % would probably be small like 5%. However i feel it would even out making it slightly useful even against graves/jihn/urgot/yi/rengar. changing it this way would just make the item not a double edge sword for rammus, malphite, and all your heavy armor stacking tanks.