Whilst I sort of agree, the reason I don't suggest that is design space.
See here's the problem with the Wizard in D&D (all editions except 4E) - the design-space of the Wizard is taken up almost entirely by two things:
1) They can learn all the spells (you know what I mean)
2) They can memorize and cast a lot of spells
They've never really had any other abilities than spells, spells, spells and more spells. They've got nothing to replace with abilities, except spells, and if you replace their spells with abilities, well, you basically got the other caster classes lol.
This is why you need a separate class for Necromancer, because you need the design-space to have, say, at least one permanent undead who does your bidding, and probably the ability to summon (on a temporary basis at least) quite a lot of other ones. You also want some spells/abilities with really strong necromantic flavour that might not fit with the general "arcane magic" vibe of a Wizard/Sorcerer/etc.
So I think unless they also dial back the default features of the Wizard - including spellcasting - there's no way to find enough space to make an actual Necromancer, particularly not without creating a situation where every Wizard is going to want a ton of Necromancy spells. Ironically there was kind of more design space in 2E because it did that - a specialist couldn't even cast certain colleges of spell (I forget how it was in 3E).
There are other classes with "design space" issues - Monks, for example, are overloaded with abilities that are default and basically make all monks into Shaolin monks specifically, whatever the theme of their subclass. Rangers needed Tashas to solve their design-space issue, by giving them abilities to replace the abilities taking up design-space. Sorcerers kind of have limited design space because all Sorcerers have to use metamagic and spell-points, even though it might be better if metamagic at least was optional.