The bleak element of Nobilis cosmology wherein the only permanent destination for a soul is the monstrous roots of the Ash is a big thing. I don't mean that it's super-important in the setting. I just mean that to imagine it—-it's big. It's weighty. It's not just something to laugh off but it's not just something to judge too easily, either.
I think it's true that the Fourth Age could change everything. I mean, in practice, we're talking about eternity in a world that is IIRC eight thousand years old; it's like an eight-year-old child confidently predicting what she'll be like at age infinity. I don't think it's that simple, though. I don't think that lifts the burden.
But I do think this burden is not the concern of the Dark; for it is apparent to them, by virtue of their being Powers of the Dark, that the fundamental crisis of existence relates to human life and not to the rooting of the Ash in corruption and suffering. To worry about the moral implications of damnation is to them a bit like a diabetic going back into a burning house for their insulin—-it'd be a very important concern if it weren't for the house that is on fire.
I think the fact that there are Powers of the Dark makes the most sense if one imagines there to be a trajectory for the Dark in any complete victory of theirs that would lead in some fashion to the vanquishing or making-contingent of the influence of Hell and Heaven both, at least on Earth; but I don't know if they would necessarily see that, or that it would matter if they did.
From "Nobilis- Justifying The Affiliations"