Here is why armour is really good against lethality.

Let's say there are 2 champions, (1) has a 1000 burst combo with 50 armour pen, and (2) has a 1500 burst combo with no pen An enemy comes who has 50 armour, right now (1) and (2) will deal 1000 damage to the enemy If enemy purchases 30 armour, they will now have 80 armour, (30 effective versus (1)) Now (1) will deal 769 damage with combo (231 reduced damage) (2) will deal 833 damage with combo. (167 reduced damage) That is why armour is even more effective than it was before lethality stack got popular.