I agree with you. That's the difference in WoL numbers and realistic numbers. You toss in a wee bit of movement and demo's dps drops off quickly. Our guild is actually using 2 locks in a 15 man, simply because we've both been playing locks for many years and refused to re-roll. He loves Demo, I love Affliction. That being said... I feel like affliction isn't in as horrible a place as it appears to be tool kit wise. I feel squishy and un-involved a lot of times in pvp which is a toolkit problem, but in pve it is a struggle yes. However I feel there are some simple changes Blizzard could make to solve the play style concerns. For example. Siphon life should be baseline and should do significantly more damage than it currently does, they could replace this in the tree with a QoL talent for all I care, I'd probably just keep sow the seeds for the utility of add killing even as a *just in case* scenario.
Soul Effigy seemed like a cool concept back when I first read it, in Beta. Now it's just a pain in the bum. I'd be happy with siphon life baseline, UA partial damage on initial application (Which I believe they're already doing) and rework Soul Effigy... IDK to something like: Soul Effigy - Passive - After two seconds, infuses your drain life with weaker souls you have reaped, increasing the haste rating of your damage over time abilities on that target by 35%.
I'm not sure on the numbers or how that would add up, but it feels more synergistic. It gives you a perk to stand and cast and penalizes you some for moving. I just want to spend less time making sure I'm in range of my al;ksdnfoaidnfaopidsnfapodsnfas soul effigy.