Iron maidens a horrible example as that fight lends itself great to destruction in my experience. Hell I 10 manned mythic kargath and did well with destruction when the bombs were being concentrated on 10 people, if you know how to move in gcds its absolutely fine.
It's the big movement where this becomes an issue, and demo will absolutely outperform affliction in those circumstances.
We agree demo doesn't sustain aoe well, the thing is the fights where you want sustained aoe are few and far between. Typically you want burst aoe or passive cleave.
Cataclysm is a great example because it fits perfectly in demos kit and doesn't really take away from it in a way that it does for destruction and affliction. Giving up chaotic resources in the other two specs for it sucks, giving up demon bolt... well we didn't take demon bolt in the first place.
The only thing it doesn't do well is sustained aoe. This is all subject to change for the coming tuning passes and what have you, but if they don't change the ratios and they get demo on par with the other two it'll be the choice for most fights.
Dunno how the t17 bonuses might affect all this.