Id like a word with your math please....
50 ARP off of 200 = 150
150 - 40% = 90 armor
90 armor isn't a lot.... but a 200 armor tank isn't difficult to build at all by the time you have reached 4k HP.
If you have 4k HP, you should have around that naturally and still not be fully built. That also doesn't include passives from mastery which can add more when enemy champs are present. Also, remember that the damage reduction scales with remaining armor, not with total armor. If you are reducing the total, the scaling effect will correspond to a lower % drop for each ARP you bring in.
Something like this(numbers are for illustration only)....
90 armor = 46% damage reduction @ 18
200 armor = 64% damage reduction @ 18
This means that the armor drop from 200 to 199 is the largest single % drop per point of ARP you will ever see, reducing slightly each point until it reaches its minimum application at about 90 AR. The % reduction to damage from 90 to 91 is less than the % reduction from 200 to 199.
46% reduction still doesn't take into account hard effects like dorans or mastery reduction to basic attacks. As well as ninja tabi reduction. If you think that as a carry you are taking out a 200 armor tank that has 4k HP from full to nothing in less than 5 seconds.... you are kidding yourself or the tank is standing there letting you full rotate while you critical every shot and get "help" from baron.
The purpose of a tank isn't to "NOT DIE"...... the purpose of a tank is to live long enough to maximize their team's damage potential and if possible, minimize the opposing team's damage potential. Usually, this is not done by "soaking" damage from a carry, although that is a partially effective way of doing it. The most effective way of min/ maxing, is to combine any CC and harass with your ult if applicable and other CC on critical opposing champions. If you look at how much "damage reduction" as a % that the tank actually brings to a teamfight, it isn't in his ability to soak damage, it is in his use of abilities and longevity to reduce or eliminate the opposing team's min/maxing.
Why this conversation keeps coming up and carries pop off at the mouth incessantly is beyond me..... it is also why I don't play ranked. I watch it plenty enough to see what works and teams without a tank or at least 2 solid bruisers usually win by the sword or die by it..... a tank provides cushion.... and that flexibility is usually more valuable than an extra damage champ.