To show the power of Last whisper items I will use warden's mail having a high % crit damage block

Omen and frozen heart would reduce damage of crits by 45% of the damage that is from crits.

So that 100 damage that became 200 damage because it was a crit instead only does 155 damage because it's damage was reduced by 45%.

It only effects bonus damage from crits in the same fashion that LW items only effect bonus armor.

And if the enemy got Infinity edge that 100 damage that became 250 damage is reduced to 182.5 damage.

And the armor part of the item? That is like the base AD of LW items.

There is no counter to % armor penetration besides "buy more health". If this version of warden's mail were in game the counter play would be "build more AD and armor pen/reduction" and that would be the "counterplay" advice we would give marksmen complaining about it.

Not saying this will happen but this is to show how powerful % armor pen/reduction actually is.