I said nothing about killing champions. I said that they were weak to burst, but should be much stronger as the fight draws out. Tanks counter burst because they can survive the initial blast, and can deal decent enough sustained damage that assassins and mages should die before their major CD's come back. However ADC's are meant to be the natural counter to Tanks, They Have high sustained damage, and should be dealing a lot of damage over time, given that ADC's Have shit scaling on almost all abilities, and inherently are made to use auto's to deal the most damage, it goes without saying that yes, Tanks should be able to be killed by ADC's, the longer the fight the more likely it should be that fully sustainable damage wins. Tanks and bruisers run out of steam after burst because they usually need a few things put together over a set amount of time to execute a successful kill. Think darius's passive, and rivens auto canceling q combo. This takes more than the 1-2 second burst a mage and assassin usually puts out, but if it goes past that, what's left? that's where the auto class champs should be punishing failed attempts to kill them, or cleaning up. THAT'S THEIR JOB, and in recent year's Riot has scaled that back, to the point that it's now almost pointless to take an ADC because an AD assassin can push almost equally as well, and can take care of themselves with all the mobility and survivability they are given.
Now the reason that Thornmail in its old form was Exceedingly oppressive is because it only affected one class, Ranged ADC's. It's not even oppressive to a Melee AD champ, only Ranged. This is because ADC's as a class MUST rely on auto's to deal damage, and the rest of their kit generally compliments their auto's to help them dish out damage, and probably a self peel or two. That's it. Melee usually has some sort of damage steriod, and it's usually in the form of Magic or true damage, giving them mixed damage that makes it hard to build against them, and makes them solid champions that melt using on hits to proc their damage steriod as much as possible. You must have the most warped mind to think that Something that needs more than 5 seconds to kill something is OP, because literally every other champ in the game can do that. ADC's are a very intelligently designed class, their job is to hang around and deal damage from a safe place, keep out of trouble long enough to clean up and push, and yes, just so you know, old thornmail was made for a time when you could build two bloodthirsters and have 40% lifesteal, you can't do that anymore, the maximum amount of lifesteal is around 47% and that's using items completely out of an ADC's appropriate build paths. Generally taking a hit for damage just to get around a completely outdated Item is beyond stupid. Thorns should help control Lifesteal, not make it so that you can stand there and watch them die because their auto's do more damage to them than you.
also just so you know, the math is wrong from OP. Thorns would do damage to an ADC according to their premitigated values, meaning if I hit using caitlyn's passive for 1000 damage. you would hurt cait for 300 magic damage, mitigated by her MR, and you would be hit for 1000 damage, mitigated by your armor. So assuming you are a Tank that built sunfire, thorns, and randuins, all of this being pre nerfs, you would have an added 225 armor, plus a base armor for a normal tank at level 12 which is about 65 (I took Sejuani's values just so you know and I won't add in masteries so you can understand the math first). That ends up at about 290, this gives you a total reduction of 75% (rounded down a percent and a half for cleanliness), giving you a whopping total of 250 damage from one auto. Now Caitlyn took 300 mitigated by her base MR of 30 which gives her a 25% reduction giving her 225 damage from that auto. If cait has a BT, she gets 20% of the 225 damage she delt, which is 45 health. Given the Effective health pool of 1475 (rounded a little down here but it won't matter because she'll still kill herself in the same amount of autos), Caitlyn will kill herself in 6-7 auto's assuming she continues to deal 1000 base damage on every auto. Sejuani here has a base health of 1740, and an additional 450 and 500 from randuins and sunfire, giving her an effective health pool of 2690, at 250 per auto, sejuani dies after caitlyn deals her 11th auto. Which never happened because Caitlyn died 5 auto's ago. Do you see why this is oppressive as all crap now? Riot Built vayne to get around this because thorns could easily make a normal tank Kill the ADC by Standing there. They didn't even have to use any of their abilities, the champs already going to kill themselves. This is exactly why thorns was nerfed the way it is right now. It's meant to control lifesteal, not kill anyone who auto's you with no consequence whatsoever.
Edit: I realise the values were a little extreme, but it's to show how powerful the old thorns was. The normal amount of time it would take for this to occur is about 15 auto's, but the tank would still have lived past the ADC anyway.
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I was still out healing the return damage on 
, 
, and 
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: Since it reduces crit damage, why shouldn't it reduce ALL crit damage? Like, 
E maybe? Fuck yeah, makes sense.
or
to build those two items either.
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The Thornmail change is a nerf when used on: 
Anybody remember how toxic it was for the champions in the last list ^ here to build bruiser-damage and just completely shutting out Marksmen by purchasing Thornmail and having the gap closers in their kits?
just because you can't kill them without killing yourself. It was nerfed so someone like 
wouldn't just get this and kill ADs without penalty, they have to build more armor now and that is what they don't do because of nerf. And YES, it was a big FUCK YOU to all poor tanks who actually used this item a lot.
