I will say this again.
Thormail will NEVER deal true damage, apply grevious wounds, or even become slot efficient. There are two reasons why. -It's not designed to let a tank deal free damage. It's designed to negate the healing of ADCs. -A champion dying from attacking someone is horrendous for the game. there is no counterplay, and nothing you could do in that situation.
The only way i could see this item becoming more relevant is if it is given more counterplay, or there are ways of countering it.
kills herself hitting
because thornmail

? Its not fun. Its not entirely efficient either, but damn if it doesnt shred the world.