Originally Posted by Anska
From the way it is described in the letters by the Gur and Sebastian, as well as in the two versions of Under Dark Epilogue that I know of, they are mostly busy trying to survive and create a somewhat comfortable life out for themselves. It is stated that some of the spawn didn't survive and others didn't choose to stay with the community.

The assumption that they become one murderous horde hell-bend on enslaving others is an odd take on this situation and on the ending that is the one about breaking free from power abuse. Of course they could, but so could any large group of people.
I don't think it's an odd take at all. Vampires
specifically hunger for humanoid blood. In any non-tribal society, an upper class will eventually establish itself, and with that comes conquest for limited resources. 99/100 times, play this precise scenario, you're going to end up with a people who practice slavery to have access to that blood. The last one is the "vegan" (animal blood) utopia where everyone either holds hands and lives blissfully in harmony, or finds willing donors. There are just too many spawn for that scenario winning out to seem likely to me.

The best case scenario is that the vampires are not particularly successful in the underdark in establishing their own society, which would eliminate this future straight at the root. That leaves the development of raiding parties, particularly among those who are not under Astarion's thumb, and drows having to deal with a vampire crisis in general. I imagine it's easier for some spawn to go undercover there than others. Unleashing 7000 spawn is a bit like unleashing 7000 mini dark urges: some will overcome it, some will revel in it. You generally want them to integrate into pre-existing societies instead of having the chance for a new tyrant to establish themselves. It's less safe for the individual vampires, but is at least a step towards preventing them from becoming a noteable faction.