While its server its purpose as an anti-sustain item, tanks no longer have any tools to punish extreme attack speed champions such as master yi, fiora, xin zhao and tryndamere...
THORNMAIL ISNT MEANT TO BE AN OFFENSIVE ITEM....
IT ISNT MEANT TO DO LOTS OF DAMAGE
[zombie-brand-facepalm]
return damage should not be a strong mechanism for dealing with enemy aggression.
"I punish you for attacking me", i.e. "i don't have to do anything. your own actions kill you". should not be a thing.
It is extremely frustrating to spend your kit on an opponent who is fighting you and see their HP go up. This happens if your champion has low damage output (see "tank") and the opponent has good sustain or specifically "pre mitigation" (hp per AA used) sustain. Thonmail's purpose is to take care of that and make sure that if you fight someone, their HP bar goes down, rather than stays even or goes up - and ideally it should go down close to what your kit is capable of outputting.
Thornmail is not and should not be for killing your opponent. You have to spend your effort and resources to kill your opponent. Not just put yourself in front of them and then they automatically die.
I build it only happen if champion x aces with armor or they have 4 ad's.
It has the cold steel passive and grievous wounds, both of which are effective vs the champions you just listed due to their kit and/or build.
