What if Thornmail procced grievous wounds ?

No, it creates a situation where there is no possible way to play against someone who built thornmail as an ADC. If the enemy tank builds thornmail, you can no longer auto them as you will just die (since your lifesteal is doing nothing at that point, plus you're taking damage).

Thornmail already does its job. It nullifies lifesteal. In reality it does more damage than the carry can heal back (unless they're super fed, in which case sucks to be you).

It's just not that great of an item because it gives only armor and no health or anything, but it fills it's niche or countering lifesteal.

With nerfs to Rammus this would be great

let's just remove all counters to it, why not

It all depends on whether or not you think life steal should be a counter to thornmail, or if thornmail should be a counter to life steal.

It felt like the latter was the case a long time ago, but currently it's more the former.

No pls

It could proc Grievous Wounds, but only as an active. I think if it lasted for 3-5 seconds and had something like a one minute cooldown, it'd open windows to use it effectively while also allowing ADCs to play around it.