doesn't flat magic pen become useless if we keep getting AD + MR items? hell isn't it already?

I personally always found it weird ADC's were allowed to get so many a AD + MR item because you got 2 forms of magic penetration:

flat magic penetration is reserved for taking out squishies.

%magic penetration is reserved for taking out tanks.

however with ADC having so much MR from AD items mages have to buy void staff to kill a squishy, does this not completely invalidate flat magic penetration?

before all these AD + Mr items you could actually forego void staff for another big damage item, sure you would sacrifice damage vs tanks but in turn you would be sure you would blow up the ADC, now however with all these AD + mr items mages are forced to buy a item that's supposed to counter tanks to be used against squishies.

I remember when ADC's had to get a banshee's veil because their damage items didn't provide any shields or defense bonuses.