If void staff is going to have 40% magic pen, then achieving 150+ MR needs to be possible w/ 2 items
I don’t know why we pretend that MR is in an acceptable state when:
Void gives 40% armor pen Flat pen is incredibly easy to acquire between sorcs/morellos (which negates half of a big ticket MR Item by itself) The copious amounts of ambient magic damage in champions passives, item actives, item passives, etc that make it incredibly easy to stack AD teams because getting magic damage from alternative sources isn’t that difficult. As such, you still can’t reallt buy MR because you need armor, but they do enough magic damage to justify at least one MR item - but the MR item is trash. Most of the %hp damage that exists in this game comes in the form of magic damage
The fact that I can invest in one MR item + mercs and still barely eclipse 100-120 MR (depending on runes) is actually laughable. Cooldowns are so low now since riot basically hands out 40% CDR for free that even conventional methods (stacking HP to avoid burst) don’t work anymore because burst rotations are up so quickly again that even burst champions are practically DPSing. Couple this with the fact that AD/crit is still the best tank killing damage stat in the game, and your realize as a front liner that you can’t justify building more than one MR item + mercs because you’ll melt to the ADC, even if they’re not fed. I’m fine with void giving 40% pen, but MR needs to not be garbage.
would like to know your location. Plus, while ad assassins may deal more damage with aas than ap assassins, their combos arent often ranged, or with the massive long range initiation of other ap assassins.
which makes the whole point of getting Magic Pen not even a big deal
, with this I can agree with you , and a late game 
who has 1000+ AP
but when it comes to other mages , they can't do shit when you build heavy MR , and if they try to use their spells on you and go on cooldowns , then congrats , they have nothing to do in the next 4 or 5 seconds at least while your team just kills them , it's not like they will burst you and they their cc isn't really that huge thanks that rune
and then a 
gets closer to you , you MUST use your spells to stop him from reaching you because an assassin or an adc from his team will finish you off
the Mao either dodges your Q or flashes it or gets hit by it but the snare lasts for less than a second thanks to Mercs and that rune that gives tenacity
Then he reaches you , cc'ed you , you got nothing to do but to die if someone on his team followed
Sire you will have a front liner too , but it will also depend on what front liner is that
Example , your Frontliner reached the enemy adc , they used their CC , they can't kill them because they are a tank , in this time when you as a mage got 4 or 5 seconds cd , the enemy adc will ignore your fornt liner and straightly goes to you and kills you , or at the very least they will attack your Tank in those 4 or 5 seconds , your Tank dies and that's it
In other words if Tanks got actually a problem , then it's the armor items , not mages ones
or 
to out burst someone like 
<--- all have MS increase btw.
once he gets a cowl u can't do anything to him, and then he finishes visage and just autowins.
like what u gonna do
+ 
= 2700 gold for 20 AP and 33 MR Pen
= 2800 gold for 450 HP, 55 MR and 10% CDR
he would also reduce most mages lackluster DPS by 20% (Tabi 12% was nerfed to 10% while Basics cost 0 Mana, have no CD and cast time in respect to 100+ mana plus spells the mage has to use and put on a CD to do dmg AND most of them are not point and click -- so they're even less reliable than finding your RMB to do his unavoidable 1000 DPS from 550 range)
= 4100 gold
cancels all but 8 mpen from both 
is its base component and 650 gold cheaper