That's a fair point. I guess that if a necromancer isn't in a rush to get his army up then he might just use his upcast slots every day, but if he needs an army now he'll let his minions get to work. Depends more on the situation.
I'd also like to point out that if the minions get their own minions, they'll be robbing us of the valuable corpses for Finger of Death. As in, it'll likely not be recommended to have them use those abilities, because the necromancer could just do it himself, and get a more powerful undead in...
I disagree with the idea that the undead is not a major feature on this spell. After all, Disintegrate, a spell 1 level lower, has higher damage AND can be used for some nice terraforming. However, this spell has lower damage, but i believe it makes up for that with the permanent undead...
I don't really think creating multiple minions with that spell would make much sense, after all it's only 1 corpse it has access to. I do like the HD idea though, but I think it could be a bit more streamlined. Perhaps limiting the CR of undead one can create with a casting of Finger of Death...
Title. I feel like 5e has neglected the undead overlords far too much, and am hoping this will help alleviate that a bit. So far what I've come up with is:
Spell Slot Level 7 - zombie or skeleton
Spell Slot Level 8 - shadow or bigger zombie
Spell Slot Level 9 - ghoul or specter...