Thornmail needs windows of power
being always-on return damage is bad design; if it's too good then it's a permanent disincentive to ever fighting them if you're based on auto attacks. if it's not good enough then it's not worth the slot.
its return damage should be juiced up a ton, but relegated to either being on an active or with a passive trigger like Sterak's. Both provide clear and fair opportunities and the item can then be balanced to feel good to buy and not cancer to play against when its good.
and stop asking for grievous wounds.