Rito has been nerfing the champions, but I believe the issue is actually the armor formula. As you get more, you get less.
That's not true, armor stays at the same value per point regardless of how much you do or do not have.
If you have 100 armor, you have 50% damage reduction, and take 50% damage or 1/2 damage.
If you have 200 armor, you have 66.67% damage reduction, and take 33.33% damage or 1/3 damage.
If you have 300 armor, you have 75% damage reduction, and take 25% damage or 1/4 damage.
If you look at HP:
If you have 1000 HP, it takes 1 1000 OHKO combo to kill you.
If you have 2000 HP, it takes 2 1000 OHKO combos to kill you, or those do 1/2 of your max HP.
If you have 3000 HP, it takes 3 1000 OHKO combos to kill you, or those do 1/3 of your max HP, etc.
With linear scaling: the %increase in power 1 point offers between 1 and 2 is massive where the %increase 1 point offers between 99 and 100 is minuscule, but 1 still equals 1 in either case.
%reductions or reduction stats in general kind of have headache-inducing powers, it's easier to look at such things as fractions or try converting them to a positive when comparing them to additive or multiplicative stats.
One thing you can do is look at the %damage taken rather than %damage reduced, as taking 40% damage to 20% damage offers a pretty awesome and easier-to-see doubling of your power, where looking at 60% reduction to 80% reduction seems kind of lame and unexciting by comparison even though it's exactly the same.
As for your proposed changes, I don't see those really addressing the key problems of tanky assassins, which seem much more geared towards assassin issues than tank item issues.
There are a lot of 50/50 defense/offense champions such a change would effect, and it's worth noting LoL isn't just about 5 major-items in end-game battles.
Realistically a lot of game-changing power is in early and mid game, and the average per-player income is 11,000 gold: or about 3 major 3k items. When you consider a lot of tanks have a rough early game, and when you consider a lot of tanks actually have no innate tankyness in base stats (Sona has better HP and armor than Shen, such a change would negatively impact tanks from really having an impact for the first half of the game.
Anyways, on tanky assassins: The main issue assassins seem to have is a complete lack of assassin-specific itemization.
There have always been pretty solid build-paths for tanks, marksman, and mages due to itemization being broken up in HP, AD, and AP categories.
Fighter itemization just got a really solid boost with a lot of great items that add a lot of choices and fighter-specific itemization to the class.
Marksman itemization got a massive boost and became really sexy.
Mage itemization took a noticeable hit in cost-effectivness.
And assassin itemization pretty much stopped existing this season.
Sure, RIot released 
: but in seasons 3~5 AD assassins had a dedicated core-build of 
that offered 55%+30 armor penetration, which comes really close to the 50%+50 needed to double damage. Considering even Sona gets 76 armor from bases, enough armor is certainly on champions to reach close to an ~80% DPS increase for such a core build, which is realistically the same bump IE+PD offered through ~50% crit with IE's crit damage bonus.
Now Cleaver requires 6 stacks instead of 4, lost the flat pen, and Last Whisper only works against bonus armor: ripping apart the assassin core-build of the past into a fighter-specific and marksman-specific item.
You get rewarded with all of 40 armor pen with a 3 major item assassin core now, which is pretty weak.
Concerning assassins: it's also worth looking at how scaling works in LoL.
Marksman are the easiest roll to look at.
If you start with 100 AD and get 100 AD, you double your DPS, from 100 to 200.
If you get 100 more AD, you increase your DPS by 50% from 200 to 300.
If you get 100 more AD, you increase your DPS by 33% from 300 to 400.
If you get 100 more AD, you increase your DPS by 25% from 400 to 500.
Alternatively: if you start with 100 AD and get 100 AD, you double your DPS from 100 to 200.
If you get ~150% AS, you double your DPS again from 200 to 400.
If you get 66.67% crit with IE, you double your DPS again from 400 to 800.
If you get 50%+50 armor pen, you double your DPS again from 800 to 1600.
With that in mind: it's also worth considering the longer a unit lives the more damage they do, and the faster you kill things the less damage you take.
Defense and offense interact together the same way AD and crit does.
And with such stat interactions: it generally holds true that out of 10 stat ponts, 1 x 9 = 9, where 2 x 2 x 2 x 2 x 2 = 32.
Why I bring this up is mostly where assassin itemization is right now for both AD and AP assassins.
And that's essentially just pure AD or pure AP.
The only true assassin items in-game ATM are and 
, and they pretty much put you in the 500 AD or 1 x 9 catagories in terms of scaling overall power.
The only incentive to build full glass-cannon on assassins is if you can 100%-->0% everything capable of hurting you in which case you take 0 damage, but so many assassins lack the tool necessary to reliably due that (largely due to counterplay), and outside of Zed: assassins are in a pretty awful spot capable of throwing the game if they fail with a glass-cannon build.
Which is what I'd attribute to a large number of assassins building 50% defense, 50% offense fighter-builds at the moment.
Defense and offense always scale together, you don't throw the game if you're beefy enough to recover from a mistake, and there is really no incentive and nothing glass-cannon builds have to offer that make them worth considering. At least outside of Rengar/LeBlanc shenanigans where you kill someone with 0 warning and seriously lacking counterplay.
That was probably a lot longer than it should have been, but you're armor idea aimed at assassins abusing tanky DPS items is a fairly interesting take on finding a solution to a current problem.
I'm just not sure it's the right solution to the problem at hand and the roots of the current problem go pretty deep.
Would be cool (when playing tanks) for tank itemization to scale alongside marksman itemization and see more of an exponential power-curve if you get ahead, without being as oppressive 1 major item in though.
vs 
Or 
