Do the math before spouting circle jerk misinformation. Thorn mail is (ironically) much stronger as a standalone item and scales exceptionally well on Rammus in particular, especially against teams where the main auto attackers aren't 100% physical DPS.
Until your enemy deals more than 167 raw physical damage with their auto attacks, the new thorn mail deals more damage than the old one as a single stand alone item, and it takes even more raw physical damage if you factor in that it now scales with his DBC bonus armor as well (they need to deal more than ~337 raw physical damage per attack to take less return damage than before the Thornmail nerf)
Even late game on Rammus it is at least as good as before since it returns what is equivalent to the auto attacker dealing 697 raw physical damage per hit if he can stack armor and activate DBC, which even against an ADC is roughly the same return damage as before on average.
Thornmail is also exceptional for boosting jungle clear speeds now since the return damage is guaranteed to be sometimes almost 50% higher than before when facing monsters.