I think I can explain this what seems to be a paradox for you.
Yes, a lot of people want Legendaries to have fixed affixes like Uniques did in Diablo 2 - because it gives Legendaries more of an individual identity and it also means that less of them have to drop - while at the same time they also wanna have item diversity.
How does this make sense?
Quite simple: they wanna have multiple Best-in-Slot (BiS) items for every slot that are competitive with each other, but do different things in different ways, so everyone can choose the Legendary they prefer.
For example, I have here an imgur album with mockups / concept art for legendary items and you can for example compare the weapons (or off-hand items) against each other and they are all more or less on the same powerlevel and each of them would be useful for every build that can use them:
https://imgur.com/a/Z5NswCo
As you can see, these kind of Legendaries did not have any massive skill-specific damage multipliers (e.g Skill X deals 500% increased damage) on them like D3’s Legendaries do, which means that they are not just useful for builds with a certain skill.
Legendary A is useful for e.g. a Seismic Slam Barbarian, as well as a HotA Barbarian, as well as a Whirlwind Barbarian, etc, and Legendary B, C and D are also useful for SS Barbarians, HotA Barbarians and Whirlwind Barbarians.
That is where the choice comes from.
Some people even would like to see that Rare and Magic Items are competitive with Legendaries in some equipment slots, like in Diablo 2 where Rare and Magic Boots, Gloves and Circtes could compete with Unique Items for these slots, but not in weapon, armor or off-hand slots.
I hope that all made some sense.