PSA: Thornmail is not - and wasn't intended to be - an offensive item
Thornmail was never intended to be a damage item for tanks like Sunfire Cape. With the exception of Rammus, it was simply meant to deter AD auto attackers and diminish or negate lifesteal. However, it was a problematic item because its return damage affected everyone, not just lifesteal/sustain champions. Because of this (and how strong it synergised with Rammus), it had to be kept pretty weak so it wouldn't just destroy everyone without lifesteal.
Now, with Grievous Wounds added, Riot was able to improve Thornmail and fit it more smoothly into its anti-auto-based-sustain niche. The damage return was reduced because it doesn't need to negate as much healing since GW is now applied. Health was added so it isn't a completely awful choice against teams with mixed damage types. Cold Steel was added to, again, improve its anti-auto-sustain identity.
You are not supposed to be able to kill any enemies who dare to touch you just by having the item. That is not enjoyable, balanced gameplay. Thornmail allows you to respond to enemies who want to use you as a walking health pot. That is its entire purpose.
On a different note, for players who want to use Thornmail/Bramblevest - it is not a generalist anti-auto item. It specifically counters auto-attackers with built-in sustain or who heavily itemize into lifesteal or other forms of sustain. Or who have a Soraka support. Please don't get it against champions who focus on crit over sustain - that's what Randuin's is for. Thornmail won't do much to stop crit stackers from shredding you.
And, a final note, because I know the main controversy over Thornmail is coming from Rammus players - yes, it's quite a bit weaker for him now, and reduces his damage output. However, rather than changing an item for just one champion, it would be better to buff Rammus' reflected damage in his kit to compensate. If he takes too much of a hit from this change, I'm pretty sure Riot will do that.
tl;dr - Thornmail is for defense, not offense. It fits its niche better now. Don't buy it against crit. Buff Rammus' reflection damage, not Thornmail's.
, but the overall developer decision to put Grievous Wounds on
This guy will two shot your ADC and laugh at their pitiful Grievous Wounds.
Juggernaut. 'Nuff said.
Your ADC is easy HP.
who build
... You get the idea.