At level 16 an Alistar using ult typically takes 25% physical/magic damage. Stacked conqueror effectively raises that up to 31% pre resistance mitigation. That means stacked conqueror enables a champion to do 24% more damage if Alistar had zero armor at level 16. Lets assume for some reason that Alistar decides to get zero extra armor beyond his base from any source. A 1000 damage strike with no true damage is reduced to 250 by the ult without conqueror, after deducting only Alistars base armor at level 16 (94.7), this is further reduced to an end value of **128.40**. Now we add conqueror to the mix: Alistar ends up receiving 230 physical damage and 80 true damage. After resistances this is 118.13 physical damage and 80 true damage for a total of **188.13**.
Congrats! In a worse case scenerio for conqueror where Alistar opts for zero extra damage mitigation that conqueror can aid in penetrating, it still yields an overall damage increase of **46.52%**. Converting only 8% of an AD champions damage gives a boost that massive _at minimum_. This is the most charitable case, any added bit of armor or damage reduction from any source whatsoever makes that guaranteed 80 damage proportionally larger. I'm not gonna write out all the details again, but if this same lategame Alistar has 100 armor from Locket, Zeke's and 2 armor shards the damage goes from 84.83 to 158.05 for an increase of 86.3%. A rune of all things is the hardest scaling tank killer in the game against Braum, Alistar, Garen, Galio, etc, and even allows for a damage output boost of **20.72-41.07%**(Depending on his level and W rank) against a Master Yi that's meditating with no armor bought, and this doesn't even get into the other benefits of the runes.
True damage is not simply "100% armor pen", it goes through EVERYTHING that doesn't outright block damage. Even only converting part of the damage into true damage has exponential implications against targets using multiple layers of damage mitigation. This rune severely cripples the synergy of % damage reduction with armor and magic resistance.