While I do agree with you having magic, rare and legendary items on the same tier will strengthen min/maxing, their current idea with D4 is to push Seasonal mode as much as possible and create a different experience with each new Season by shifting the meta game.
The above could be more properly done with their way of tiering the itemization aka magic → rare → set → legendary, because legendary items have special affixes and the meta shift will be greater to these than to the items in the other tiers.
What I am trying to explain might be hard to understand, but it is the right design philosophy if you aim to embrace Seasonal mode on all costs.
Their current version of trading also confirms this → dedicated NS players will be left behind, because they want to push Seasonal gameplay with new and fresh mechanics. The best way to achieve this is with THAT type of tiered itemization.