Before anyone down votes this thread, what OP said is true.
The nerf to LW to penetrate % Total Armor to Bonus Armor hurt AD champions a lot. Considering that most champions have upward of 90 armor late game, AD champions lose equivalent of 36, if not more, flat armor pen at level 18.
Armor scales in a logarithmic function in terms of the % damage mitigated vs actual armor value. For each point of armor, you receive 1% less physical damage than before. At 100 armor, you receive 50%, at 400 armor, you receive 25% damage.
To elaborate, it is 1% of the damage you would have take given your current armor value for each point of armor. So it scales less and less because you would take less damage.
Generally, Logarithmic function is bad, it takes 4 times more armor to reduce 2 times more damage. However, armor is in a direct relation with health. The product of these two stats leads to some extremely high values on tanks.
Without armor pen, it might take an ADC with a generous 500 AD around 30 to 40 autos minimum to kill a tank with max armor and around 4k health.
That is why people "complain" about the tank metas.
To tanks players, it might not feel that overpowered, but to other players in the game, there is literally nothing they can do to the tanks.
Without
,
,
, or
who can deal % true health damage or shred tanks with AP (Void staff still calculates from total MR), there isn't much that can be done to tanks. Realistically, no tanks are going to just sit there and let you wail on them. Imagine that level of effective health on a champion such as Sion or Volibear.
Black Cleaver helps, but it often requires someone else on the team besides the ADC to build it.
Resistance should always counter flat penetration, but it should not also counter % penetration.
The tank players who argue that this mechanic is healthy are just hypocritical. Tank players complained about penetration being too overpowered in season 3/4 while assassin players argued that it was justified.
Now the tank players are the ones defending an unfair mechanic.