That's like saying you don't really need a microwave because you can just start a fire to warm up your food. Sure, all you need is some wood and some matches. But a microwave makes it a lot faster, convenient and pleasant.
Same with gear. You might be able to clear Torghast and world quests with crappy gear. But that doesn't mean that better gear isn't still nice. Gear exists as a reward and the main character power progression mechanism at max level since the start of the game.
It's extra silly when you consider that the things you consider to "require gear" are arbitrarily created by Blizzard.
Also in some cases it's simply not true. The first +15 key was cleared by people in
~183ilvl, therefore M+ gear is redundant.
Mythic was cleared by people in
~220ilvl, less than what Mythic drops, therefore Mythic raiding gear is redundant.
And PvP gear is only necessary as much gear your opponent has. Gear could be completely removed from PvP and it would still work and be competitive.
It's a bullshit gatekeeping argument that just draws a very arbitrary line on what gear is "necessary" or not. Gear is
the main endgame reward the game is designed around, therefore it should be rewarded from every endgame activity.