"I disagree that all people with DS suffer in the way that you describe, but let's run with that."
Again, I'm not saying they do, I'm saying it sounds to me like that's in the ballpark of what Dawkins is implying and thus basing his argument's moral framework on.
"Should all potential people who are likely to suffer be aborted? The poor suffer from poverty, women, gays and black people suffer from discrimination, everyone who has ever lived has suffered at some point, perhaps the moral position is to abort every foetus?"
You give the difference in your description though--
"Potential" for suffering vs. guaranteed suffering.
There is no guarantee of poverty, discrimination, etc. at the point of birth...being born in a certain place or time might increase the likelihoods of that, but that's hardly reason to abort a healthy fetus, as that suffering won't be intrinsic to the potential new life, but extrinsic.
By contrast, the suffering WOULD be intrinsic to a life born with a debilitating illness, and it would be guaranteed insofar as the condition is guaranteed.
Say Disease X makes all people born with that condition suffer agonizing pain for the totality of their existence, but they're otherwise totally healthy--is it moral to give birth under such circumstances and allow for that degree of intrinsic, unending suffering?
To go a bit Dostoyevsky for a moment, it's arguably not the actual suffering that's the terrible thing, but the GUARANTEE of suffering, and that suffering continuing as long as you yourself continue...to use Dostoyevsky's analogy from "The Idiot"--bad translation of the Russian title, probably, but that's Barnes and Noble's fault--if you were condemned to be executed, it wouldn't be the actual guillotining that'd be the worst part, but the days and hours and seconds leading up to it and KNOWING you were going to be executed...
And that such a fate is *inescapable.*
You can escape poverty, or flee from discrimination...
You CAN'T flee from the ills that ravage your own body, which is what makes that kind of suffering a special kind of hell--it's not just potential suffering, but guaranteed, and you can't ever get away from it, because "it" is part of your very existence.