Dear RIOT, Thornmail is now countering mages more than auto attackers.
No, no it's not. Even with liberal use of auto-attacking: a mage isn't going to deal much of their DPS in auto-attacks.
Auto-Attackers have a much larger portion of their AA damage in auto-attacks.
Also - Thornmail reflects essentially the same damage to realistic levels of AD with realistic levels of armor as before. 15%+25% bonus isn't any different vs. someone like Riven than the 30% was before.
Main change is it sucks more vs. critical strikes and Sheen, but now it punishes on-hit stuff like Kingblade %HP shred, Vayne True Damage, Teemo Poison, Devourer, and so on.
So rather than only being good vs. Fioramat, it's now decent vs. pretty much everyone that auto-attacks: crit-reliance or on-hit.
And good in a larger amount of situations > good in only 1 situation.