Ok, let's figure this out. Let's say our options are +50% bonus damage (old IE) vs 10% of the damage being true (new IE).
Let's simplify and say 10 true damage vs 50 physical damage.
The way armor damage reduction works in League is calculated like so:
100/(Armor+100)*Raw Damage=Final Damage
100/(Armor+100)*50 Phys= 10 True
100*50/10=Armor+100
500-100=Armor
400=Armor
This means for that 10 true damage to be worth more than the 50 physical the target will need 400 armor.
Nobody on that list is ever going to have over 250 armor on a meta build much less 400, which means every single one of them benefits from ADC's dealing true damage on crits instead of just dealing bonus raw damage. The final calculation for the actual values of the old IE vs new IE is a little more complex because of the differing values of the crit vs crit multiplier, and the AD differences, but the point is that in this case the true damage is directly beneficial to everyone on the roster who is not an intended target.
If you think that ADC's are still killing squishies too fast, then the solution is to move even more of the power into true damage. Nerf the AD by 10 and up the true damage on the passive, it lowers the breakeven armor point, but it decreases their damage overall. Their damage towards high armor targets will hopefully remain the same but the damage to low armor targets will be decreased.