Talks about cherrypicking, proceed to cherrypick demo mythic helya at a shitty 75th percentile.
Demonwrath is not a reliable shard generator on a single target with movement, period. You cast it because you got nothing else better to cast. It's like saying if shadow priests were able to cast mindsear on the move that they wouldn't suffer with damage on the move without StM because they can fill empty GCD's with a shitty aoe spell as a consolation prize.
Demo is a turret spec and suffers from movement. It loses a lot more from movement than other specs. You point out other ranged specs falling below demo but that's misleading because their issues are numbers ones. BM hunters lose little from movement but their problem is their spells are tuned like shit. Mages got wrecked. The only casters who are doing particularly well are shadow priest followed by warlock.
The question is not whether demo beats other class specs but how much of its potential damage it loses when it has to move. There's also the problem in how most demo locks play, which is that they sit on a boss (like odyn) and ignore priority adds while the other classes do the work so they will always show high as a result on boss damage because if they tried to do add work/target switching they'd plummet.
It's not uncommon with Helya that during the opener the demo lock just tunnels his thal'kiel setup on Helya after she's already used breath and while the other ranged are already DPS'ing slimes the demo lock is instead pumping out his TKC on Helya.