My guesses why Aff is there where it is right now.
1: Fixing player distribution across WL specs.
We’re coming from Legion where Aff was the go to spec in m+ and for the most part in raid, while destro was superior in a couple of boss fights, but demo nowhere to be seen.
Now, in BfA, someone at Blizzard said: Make WL speccs more even! (which is no bad thing in theory obviously.) They wanted to fix demo’s inferior role by taking away AOE from Aff and giving it to demo, and we end up with 2 crippled m+ specs (no baseline kick for demo, no AOE of Aff.) And when trying to figure out how to remove AOE from Aff and buff Single Target they came up with the Dark Glare/Deathbolt/Haunt idea that, according to my perception, NOBODY likes.
But it’s not enough to just argue against this new rotation, because, to be fair, you could argue against it and say “okay, what was fun about Legion’s drain soul cycle?”, and to answer that we have to look at what else we lost after Legion, which is shard sniping for example, or finding opportunities to trigger Sephuz etc.
If you look, for example, at the Antorus Mythic High Command fight, it was this kind of fight and playstyle mechanics that made Aff SO MUCH EFFIN’ FUN.
2: Fixing player distribution across m+
In m+ high keys Aff was the most played DPS class. In order to remove Aff players from m+ they needed to make Aff/WL in m+ weaker compared to all other classes. They chose the safe way and just removed AOE. Easy.
IMO this is why we are here.