Thornmail too weak Randuins unhealthy
counters
way too hard.
too.
The reduced crit damage is not unique.
6x
?
Okay. 
As for
A tank will likely have 200 Armour max 50 magic damage from armour is way less than 15% of a premitigated crit that could've been 1000 damage, or 150 reflected damage from thornmail pre resistances. Thornmail also isn't good unless you have health to back it up, so the scaling with armour is counter intuitive. It does way less damage, on top of how it already is reduced by magic resistance. Most lifesteal items will let you outheal it.
is 100% countered by
ever since 2009 and guess what, we all survived. And also crit is not even healthy for the game since it is pure luck. So having some sort of defense against it had to come sooner or later.