There's a lot of misconception here!
There's nothing 'wrong' with Demo in numbers other than 'input-for-reward' issues especially internal for Warlocks but also versus other classes; which they 'hinted' they would do something about.
The problem with Demo is that everything it does is clunky for no apparent reason and it is tuned around these theoretical circumstance(s) that is most likely not going to happen - which means anything not corresponding to this will result in sub-par performance and thus Demo is unpopular for the masses.
For example,
the two significant output abilities for Cleave and AoE:
- HoG requires add-pack timing and matching to your 15s/ 2 charge(s) - It is very strong, but that is only to match it's very low up-time.
- Any relevant/ class competitive AoE on demand requires Fury and MF (Immolation Aura).
(Which means sometimes for very short lived packs your AoE is seemingly best, but when you cannot time it, your AoE is the worst - which averages out versus other classes on average - yet Demo max performance is among the other classes (which is unfortunately what it is tuned around), yet this is most likely not going to happen on the average attempt, assuming all attempts are subject to unknown RNG.
Blizzard seems to be completely unaware of this, does nothing to solve it - in addition to the short range of your general AoE abilities and clunkiness. But Demo also loses it's niche in WoD with the removal of Carrion Swarm and giving the other specs matching burst AoE capabilities on talent choice.
Which ultimately, leaves Demo with nothing.