I can only see global tank-item nerfing work if we remove %Pen

For the Tank update, I mean. The plan right now, as I understand it, is to scale back the resistance value on defensive items by some amount while at the same time increasing the base resist growth of Vanguards, Wardens, and probably Juggernauts.

Problem with doing this and nothing else, however, is that it could potentially make any form of defensive itemization on otherwise squishy champions even more of a trap. If you think being an AP Mage with only one armor item is bad now, think of what it'll be like when its armor values get dinged by 20% with no compensation. Now it'll be even easier to hit you for true damage between lethality and %Pen.

That's why I think Riot should finally take the opportunity to remove %Pen and %Shred from items completely. Combine with a nerf of about ~20% to resist from items, that should solve the enduring issue the game has had for quite some time now. If you're up against a hard target and you have no %Pen they're effectively immortal. If you're up against a hard target and you DO have %Pen, they go down like a sack of bricks. However, if we remove %Pen/Shred and nerf resist down to sane values the situation isn't nearly so binary.

That also means that a defensive item or two would be less of a trap option on, well, basically everyone else. Because I don't think making it counter-productive to build defensive items on champs like Yi, Xin, and Wolfy would help their viability at all. There are, after all, plenty of champs who want to build defensively who aren't Tanks.