Lets get something straight about Lethality.

Lethality is flat armor pen, Black Cleaver and the Last Whisper items are not Lethality.

I hate when people group Lethality in with BC and LW, because they are not Lethality, one is %Armor Reduction and the other is %Bonus Armor Pen.

Armor does indeed counter Lethality. BC/LW counter armor.

Now the real problem is not Lethality, it's Lethality combined with the other two items. No one complained about Flat Armor Pen prior to this year (that I can remember), so why do people complain about Lethality? The answer is because people assume BC/LW are also Lethality, when they're not. Again Lethality is the old Flat Armor pen, except it has to scale up now, to compensate, all 3 Leth items have bonus out of combat movement speed.

Now how do we fix this? Well Riot could make it so Black Cleaver and Last Whisper's passive can't stack. This would probably also mean that Black Cleavers Armor Reduction can't be applied for every champion (Ex - If Darius gets all 6 stacks of BC on Garan then Garan's armor is only reduced for Darius instead of Darius and his allied Graves and Ashe.).