Do we really need Adaptive Helmet?

So, I might be biased as I play Swain a lot, but I also play tanks a lot(tanks-tanks like Naut, Poppy and Mao, and juggernauts like Yorick) and IMO - and I stress IMO - the problem with sustained magic damage vs tanks is not how effective sustained magic damage is vs tanks.

The problems are a) there is so much armor penetration in the game ATM that if you invested in 2 MR items you will pop like a balloon, as most teams will have 3 or 4 physical damage sources often ignoring as much as 2/3rds of your armor.

and b) most MR items have poor/no synergy with many tanks. SV suffers A LOT if you don't have a form of sustain, and is much more of a fighter item. So is BV - blocking one spell is crucial if you are relatively squishy, it gives nothing to a tank. Sunfire/gauntlet/omen/deadmen are all much better items, and buying them as i.e. Naut or Poppy makes you much more of a threat than getting early SV.

Now, some people might say that AD is supposed to be better at tank-busting, but when we have champions like Rengar, Shaco and Graves running rampant one-shotting squishes, I fail to see the logic behind gutting anti-tank magic damage sources when they are clearly inferior to AD tank busting - tank are simply forced to invest so much in armor that they can't afford MR.