It doesn't matter if you can, eventually, do it. Even people who aren't in top end, bleeding edge guilds like to compare their performance against their peers and try to play their class at a high level. The way the system was designed in Legion made it very, very frustrating for people who were doing what they needed to every week to maximize their chances at an RNG legendary... and then getting a shit tier one. It also encouraged grinding for a game that had been, for the better part of the last 10 years, a gear up to do the first raid and then all you need to do is log in for raids each week kind of game. Legion turned that on it's head.
It's very very easy to see why so many people would be frustrated with it. And the argument isn't whether or not it was doable, it's about frustration levels regarding how RNG it was. Min-maxing is fine, it's a carrot, it's something people enjoy thinking about. Grinding is even fine. Grinding for a low percentage chance at a game-changing item is not fun. Even if you get the right item it's relief that washes over you, not excitement. That's bad design.
And this is coming from someone who played a class where Legendaries didn't really matter because I was god-tier practically naked (Guardian Druid) AND I got two of the BiS legendaries right away anyway. And as I recall you played a Guardian Druid as well didn't you Fyff? Pretty easy to enjoy the game when you're OP no matter what.
I enjoyed raiding until NH where my guild's lack of logging in outside of raid started to effect performance and the amount of time we spent wiping. I got so, incredibly lucky this expansion. Picked an initial class that was OP and got great legendaries. Then switched to another fun, OP class (Frost DK) and immediately got the BiS legendaries for that spec as well. But I can see why people hated it. And it really sucked doing almost twice as much damage as our lowest DPS in a mythic raiding guild and not being able to tell right away if it was just their class and lack of legendaries or if it was because they fucking sucked.