The effectiveness of Armor Pen stacking is simply a symptom of a larger issue.

That issue being that Armor is way too good right now. There's too much of it available in items, those items have too strong of secondary effects to go with their stats, and champs gain too much armor per level naturally. The fact that this is a viable core build on AD casters and assassins is a testament to just how strong armor is; elimintating as much of that armor as possible is pretty much the optimal strategy. You want to make an argument that armor pen as a whole is too strong? I wouldn't necessarily disagree, but it's important to recognize that these kinds of builds are simply a response to counter armor stacking. If you want to nerf these items, you're going to need to nerf armor across the board as well. in particular is the biggest offender, being both extremely effective and gold efficient at the same time.