No, thornmail doesn't need grievous wounds.
A tanks job is to either disrupt the backline or peel for carries, NOT to kill them. And juggernauts have enough damage to kill adcs through their lifesteal anyways.
Thorn mail DOES need a rework though, it's 100 armour isn't really good in most scenarios.
What it should have is the following:
300 hp 75 armour Unique passive: deals damage equal to x (+ 15% of incoming damage) to attackers as magic damage.
This way armour stacking doesn't increase the effectiveness of the item, and allows it to be a better item on its own. Also doesn't allow niche cases where it's much better on some champions than others.
that's slightly more effective against the right opponents, and Thornmail's overall intention is in a pretty similar place to the cape.
It's not supposed to be a miracle item that hard-counters an 11,000 gold build on a carry, just a defensive item that provides a nice chunk of defense and a smattering of damage.