I'm not saying that an ADC will get it early. That's irrelevant. The point is that I can get it on an ADC (any one of them, not just Vayne), and it will give me every defensive stat that I could possibly want.
Let me make my point clear. I'm saying that the problem with your item is that it isn't just a slot-efficient defense item for tanks. It's a slot-efficient defense item for absolutely anybody, and it has nothing that actually really necessitates that a tank buy it. It is the absolute perfect defensive item to fill the fifth or sixth slot on an ADC, it's the perfect defense item for a bruiser, it's the perfect late-game defensive item for an assassin. It serves anyone who just needs something to make them tanker. It has nothing that actually works well with tanks, and this is a major problem because a major issue tanks have is that other classes can use defensive items better than they can.
The problem with your item is that it is just a huge slot-efficient chunk of defensive stats. Every single class in the game wants that. I'd love to be able to fit more defensive stats into a late game ADC build, and I need it more as an ADC than as a tank because I can only really get one defensive item. GA passive also helps squishies a ton more than tanks because squishies are more likely to have people overcommit to kill them and they are more likely to get picked off.
Your item would make squishy classes a lot more than it would help tanks because it increases the amount of defensive stats that they can have in their only defensive item by a huge amount, while the difference that having one slot efficient item will make for a full-tank is much smaller.
If you want to have an "Infinity Edge/Death Cap for durable champions", then it needs to be like Infinity Edge and Death Cap and give a defensive stat multiplier or some kind of effect that rewards you for having defensive stats instead of just giving you a huge chunk of them.