I mean, it took an entire expansion to reach that point. The Legion iteration of Demo was a dumpster fire, casting Demonic Empowerment every 2-3 GCDs just to make your demons not completely and utterly useless.
Also, Aff is as dominant as it is in BfA (and Legion) because of how oppressively (and how consistently so) it has been tuned, and because frankly it is more mobile than the other specs (and for Legion at least, had superior utility, since specs were more locked into pets, and Felhunters are usually more useful than Imps or Felguards on the utility side). In Legion especially, Aff had incredible single target, incredible cleave, and absolutely jaw-dropping AoE. In BfA, they lost that AoE potency, but their single-target and cleave has remained, and they gained a remarkable amount of 3-minute burst with the introduction of Deathbolt + Dark Glare.
Basically, they are as dominant as they are not because most warlocks prefer Aff over the other two specs on a gameplay or thematic level, but because Aff is simply the only valid option in raiding content, and has been for roughly 3 years now.
Prior to Legion, it was a different story. Charred Remains Destro was extremely powerful in Legion, as was Demonology pre-kneecapping. Both Destro and Demo were also quite strong in MoP (MoP was arguably the heyday of warlocks, tbh. MoP’s iteration of both Demo and Destro was exceptional). So memories of warlocks only playing Aff is rather selective and/or only recent (well after Demo’s kneecapping around the end of BRF).
This is, frankly, naive. Of course the devs play WoW, and I can pretty much guarantee you that every spec in the game is played by at least one of them. It’s also rather naive to think that they have their “favorite” specs, personally, and will dumpster other specs while giving special favors to their own. It was a meme for a long time that the devs played mages, which is why warlocks kept getting shat on…until Aff dominated for literally 3 years straight, and mages could hardly get a couple coins in their pity jar.
The devs have also said before that they don’t have specific devs (or even subteams of them) assigned to specific classes, and that development of all of the classes is much more communal than that. It’s conceivable that certain devs, due to preference, might advocate more strongly for certain specs when the team is deciding what needs worked on, but it’s highly unlikely that those preferences are universal amongst the team.
These forums have a talent for assigning all of the worst personality traits (capriciousness, selfishness, laziness, mythomania, etc) to the devs, both as a way of voicing that player’s frustration and as a way of attempting to explain the unexplained decisions made by the devs (following the same habit as most of human superstition, actually).
Anyway, don’t take this as me defending Blizz or white-knighting them or something. They make a lot of decisions that I don’t agree with. I just take issue with people hyperbolically demonizing them. It doesn’t help one’s case and just makes the forums that much more toxic for the devs, reducing the likelihood that anything out here will make it to their desk.