Succubi are tricky because - in 4e, at least - devils and hence fallen angels.
But anyway, just as killing Tiamat should reduce the amount of greed and envy in the world, so killing the Queen of the Succubi should reduce the amount of lust and sexual wrongdoing, shouldn't it?
These are, of course, very personal-taste questions. However, for my part, the difference is in the nature of the reduction. Killing a succubus, even the Queen of the Succubi, instrumentally reduces the amount of lust and sexual wrongdoing. Whatever plans she might have had are now gone, her active efforts to keep the world lusty are now absent, her servants will at the very least be distracted by the hierarchy disruption and more likely fall into infighting and competition. But her death doesn't cosmologically lessen the
essence of lustiness, because, not being a deity, she doesn't have any direct connection to it.
Now, in fairness, killing Tiamat (or Bane, Bahamut, Erathis, or whomever else) absolutely does imply most or all of those instrumental effects. But, because they're deities, killing
also them has an intrinsic effect on the things they, in some sense, "are." It's not just that beings that
create lust or hope or whatever are no longer creating it, it's that, by killing them, "lust itself" or "hope itself" is actually diminished. Ordinary people, even those who don't have any association with these beings, will exhibit less lust or less hope or whatever. It's basically impossible to kill these underlying concepts
entirely, of course. Still, killing these beings, that in some sense "are" those concepts(/parts thereof) endowed with sapience.
In somewhat crude terms, in killing the Queen of the Succubi or the Lord of All Angels or the Ur-Slaad, you kill a
champion of the concept they represent; in killing Aphrodite Pandemos or Bahamut or Tiamat, you kill part(s) of the metaphysical concept(s) they "are." I hope that better communicates what I mean--the difference between "there are fewer reasons/temptations to do X" and "X itself now has less influence on the world, even where it already existed in full swing." Angel-lord or Succubus-queen killing is only instrumental; deity-killing is instrumental and intrinsic both.