But we're specifically talking about Uu'nat here & if you understand the mechanics it will never truly be farmable. It is an encounter where one individuals mistake is pretty much a wipe.
For guilds who just run normal raiding hours this will take weeks to down even in it's current state. The top end guilds now are probably on about 110 hours on this boss, you work that out into a normal 4-5 hour a day raiding guild. Say 5 hours per day, that's 22 days, over three weeks of continuous play every day for 5 hours.
Gear up from dungeons & craft? These guilds literally do multiple clears to trade loot which is essentially the same thing & have about 3-4 max level characters to maintain in order to help the gearing process. If anything the loot trading makes it more optimal to funnel gear to the characters that need it.
Your argument really isn't well thought out as this boss is still very difficult & you clearly don't know the sheer level of time these guilds put in to this badly designed game. Doing that AP grind in islands was simply because it was the most efficient route to gain the levels on the neck needed to make the gear you got from the raid actually you know... viable and wearable.
You're referring back to resist gear which imo was a very long time ago which ended roughly after the second expansion where the whole raid needed some resist gear, again a poor game/encounter design merely to prolong content consumption and nothing more. Now you can work your way through each difficulty if that is your thing not like the old days where there was simply one difficulty.
This raid Crucible of Storms was simply timegated which = your attunement. Raids were released with the patches and you had to attune, now the patches hit with the raids but have an opening date. The only difference being you simply need to wait rather than do a bunch of quests.