The actual math behind new thornmail
#Old thornmail: UNIQUE: Reflects 15% of the physical damage taken (+ 25% bonus armor) from basic attacks and on-hit effects back to its source as magic damage (Calculated before any resistances or reductions are taken into account).
#New thornmail: Unique Passive - Thorns: Upon being hit by a basic attack, reflects magic damage equal to 10% of your bonus armor plus 25, inflicting Grievous Wounds on the attacker for 1 second.
At what point is the old thornmail doing more damage than the new thornmail?
0.15 * RawDamageReceived + 0.25 * BonusArmor > 0.1 * BonusArmor + 25 simplifying 0.15 * (RawDamageReceived + BonusArmor) > 25 simplifying RawDamageReceived + BonusArmor > 166
So add up the raw premitigation damage and your bonus armor-- if that value is greater than 166, then the old thornmail does more damage back to the enemy than the new one will.
How much more health can you expect the most focused auto attacker to gain over time with this change, comparing the old thornmail to the new one?
The amount of health in either case is equal to: AfterMitigationDamage * LifestealPercentage
But we can use this equation: AfterMitigationDamage = RawDamageReceived * Mitigations
To incorporate the inequality above: AfterMitigationDamage / Mitigations + BonusArmor > 277
Mitigations can come from any source, be it damage reduction or armor. For example, 100 armor is a mitigation multiplier of 0.5, 200 armor is 0.3, 300 armor is 0.25 and 400 armor is 0.2
What this says is that the lower the multiplier due to mitigations, the greater the value of the expression AfterMitigationDamage / Mitigations, thus the greater the left hand side of the inequality, which stands in favor of the old thornmail.
To figure out precisely how much more healing you can expect, we can plug in some numbers. Lifesteal 20% Raw Damage 800 Bonus Armor 220 Total Armor 300 Damage Received without armor pen 200 Healing without armor pen 50 Damage Received with last whisper 328 Healing with last whisper 65.6 Old thornmail damage: 175 New thornmail damage: 47 DIFFERENCE: 128 per crit with these values
In the case for the old thornmail, you are doing 125 net damage to the enemy after their lifesteal is taken into account (with no enemy armor pen), or 109.4 net damage if the enemy has a last whisper. In the case for the new thornmail, the enemy is taking 17 net damage without last whisper, and 7.4 damage with last whisper.
#Thornmail strengths have been shifted from a super focused cheap anti auto attack item to something resembling an offensive version of randuins omen.
Riot is trying to kill the player expectations of thornmail being such a powerful item versus auto attackers that it does damage well above and beyond the enemy's lifesteal.
To put it simply, RIOT wants it to be possible for auto attackers to auto attack tanks and either not lose a significant amount of health with a standard amount of lifesteal, or gain some health if they stack a lot of lifesteal, regardless of what items the tank has or whether grievous wounds has been applied from another source.
#In the majority of games this simply means that the new thornmail will "turn off" the health drain from an auto attacker when the auto attacker is attacking you-- it won't do any real net damage to the attacker the way the current thornmail does.
I won't speculate as to whether the game would be better off with or without; but i suspect that if this change goes through other changes will need to be made to make lifesteal a unique passive to prevent stacking it.
