Lets say you have someone on the enemy team with last whisper.
They are melting you.
There are a LOT of armor items that give you 50+ armor. And a few that give around 100.

What I am saying is, if a AP Mid buys void staff, there isn't really much MR to buy.
Banshees, FoN, abyssal...
The only 3 that I can think of that give 50+ MR.
The rest of the MR items aren't really worth buying because of the low amounts of MR they give.

Add more items that give 50+ MR? Nerf void staff?
Idk.
Thoughts?

AP Mid usually throws out 3-4 spells (depending on kit) for about 2-2.5k raw (as in unmitigated) magic damage every 8 seconds.

AD Carry is throwing out 2.0+ attacks per second that deal between 250-500 damage per hit after armor in most cases. That's 16 attacks before the mage can throw out another volley.

Have you ever played ARAM and stacked some magic resist?

You will see how those few MR items can make you invincible against those casters.