A Question about Lethality

Hey everyone, I've been thinking a lot about lethality, armor penetration, etc. with the current update and now I'm aksing myself: Wouldn't it be better if lethality is scaling with the enemy armor instead of their level? Since defensive items are cheaper than offensive ones, a lead would be less rewarding when the enemy is lower in level but is buying armor because your armor penetratioin kicks in when they reach a certain level (which is delayed when they buy armor). I mean with lethality your armor penetration scales linearly with the enemy level, but the efficiency shouldn't be linear as soon as they start building some armor. If lethality scales with the enemy armor, it shouldn't make that much of a diffrence in the first place because the amount of base armor increases with level so lethality still scales with the enemy level, but it will be more usefull when they start building armor. I guess that way the hard snowball effect is still stoped and it's worth it to buy lethality when your lane opponent starts building some armor.

There may be some mistakes in my argumentation so I'm open for a Discussion :). And sorry, my english is not perfect and I hope I'm in the right Forum here.