A rhetorical question so I can understand your view...
A high level necromancer would lose Animate Dead and replace it with something more powerful? What if they wanted to merely animate skeletons?
Thanks.
You did not ask me, but--at least if I were the one writing it--I would expect that at the level you lose access to Animate Dead, you would have the option to pick up Greater Animate Dead if you want it. And at the level you'd lose Greater Animate Dead, you could pick up Mass Reanimate (raising
many of the weaker undead) and/or Create Undead (allowing you to produce the strongest possible individual/small set of undead minions). In other words, if there's something that's supposed to be a pretty core tenet of the experience, it should grow with you, not just remain frozen in place.*
More or less, your abilities
intensify once you cross certain thresholds, rather than solely expanding. This helps simplicity by restricting the total number of simultaneous options, while still allowing growth. It is, however, still some complexity cost and not none--because needing to change your abilities is a complexity increase, just less of one.
*Alternatively, make it a core class feature that gets updated inherently. Don't
let the Necromancer
not have some version of "Animate Dead", because it's meant to be part of the experience.