The Thornmail changes prevent everyone except FULL TANKS from building it defensively
So assassins that sacrifice their single non-offensive, non-boot item slot can no longer have a half-decent effective way of mitigating (and returning) a bit of damage on their primary targets?
Chasing down an adcarry when I'm Darius, desperately trying to get a pull, and no longer having my Thornmail + Randuins combo? Now I just have Randuins. And it's nerfed.
Thornmail wasn't just a niche item. It was helpful as a last purchase for most assassins, and a midgame purchase for most bruisers, to be able to do their job without getting two shotted.
If I'm playing Zed, how do you expect me to jump on Graves when he can Autoattack me ONCE with IE, then ult me for 90% of my health? Assuming this isn't a teamfight and I wasn't hit by stray AoE (which is extremely common, mind you) I would have less than a second to be able to try to kill him, just because I no longer have a flexible defensive item in the form of Thornmail.