But why? Why is that flawed, what makes it wrong? The Tank doesn't do enough damage to actually kill anything, so there would be no point to jumping into the enemy team with no follow up from your allies. So why shouldn't they be able to survive?
In certain scenarios, I agree. A Tank shouldn't be able to just exist underneath an enemy tower for ages without heavy investment or specific circumstances (Like Alistar's ult, for example). But in a straight up teamfight, a Tank should be able to persist within the enemy team.
Moreover, I think you and I agree in actuality. You're right in that other champions will take damage off of the Tank, which indirectly influences the Tank's durability in teamfights. I might be misunderstood, but I'm not implying that a Tank should last upwards of 10 seconds when the enemy team is actually focusing them down. I'm saying that Tanks, as they exist right now, are not overpowered and probably survive just the right amount that they should.
Either way, we don't need more Armor shredding.