Help me understand the Armor/MR changes

Armor and magic resist prices have now been normalized at the top and mid ranges while armor is more expensive per point at the lowest item tier.

As someone who hates dealing with Leblancs and Dianas, I'm happy about the change. On the other hand, as someone who enjoys playing Zyra, Cassio, Syndra, Ori, Lux, Elise lane, etc., I can't help but feel that the higher MR price was balanced around recognition that pure mages have to invest tons of gold into mana sustain in the early game.

Against AD assassins and fighters, you also have to heavily weigh move speed and armor in early item selection. Now the Zeds and Yasuos are going to be tankier earlier and for cheaper. While you're building 6000 gold to have mana and not be 1-shot in an ulti, they've already finished their Maw, Merc Treads, and Botrk.

Seems like an unnecessary change. The champs I mentioned are just going to have an even harder time doing anything to the infinite sustain AD champs than they already do. Oh, so let's buff Zed.