TL;DR loot is core to Diablo; undifferentiated loot adds to the existing bottleneck of tedium of inspecting trash loot, and misses some of the “cool”.
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Your pushback is usually generous, but I think this one’s a miss, hear me out. 
I don’t think it’s barrel-bottom like the “cultural” stuff, because it goes to itemization. Itemization in turn, dominates gameplay. Having differentiated armor types adds layering, enhances class fantasy, and (my biggest bugaboo with D3 & D4) makes interacting with loot much more efficient & enjoyable - without employing a loot filter. Once they relent & add a loot filter, this main problem goes away, though the fantasy still suffers.
The reason D2 base itemization was so satisfying, probably without people realizing it until D3 took this away, is because you almost always knew if an item was worth picking up to inspect it, and how likely it factored into your plan… before you ever picked it up, much less read the tedious stats. So your time was rewarded… in the career of one character, those seconds become minutes become hours. The tedium itself, over those cumulative hours, is pretty atrocious (in the realm of video game interaction, which is separate from real life woes ofc).
Like with several other bad-move detours (probably influenced by a roadmap that included consoles & phones I guess?) it was probably Jay Wilson’s bad idea. It felt yucky then, and it feels yucky still. 
I currently don’t play any game as much as D4, so I’m proof this isn’t just opportunistic dumping on its flaws. I want it to be good - sadly, base armor differentiation is probably never coming back to the franchise. 