This is absolutely not what I'm suggesting, as I think you know.it is more why let everything be a necromancer when we can just make one really good one?
My initial comment was about the design space that having subclass levels balanced across classes opens up. And then I gave an example relevant to the thread, where (for me) two classes both of which cast lots of spells within the necromancy school might choose to specialize in them.
This is relevant, because (apparently) the new PHB will only have four wizard subclasses, which means the previous specialize-by-school approach won't work the same way (some have suggested a school-specialist; that's of course still possible, but I think it will disappoint because of the range of things offered currently -- e.g. the level 6 abilities are all over the place, and would take up a lot of real estate if each were spelled out under a given subclass).
I like the idea of necromancers being either clerics or wizards; you don't -- that's fine. I've explained why in the history of the game I think that's valid, and that in a previous edition clerics were better necromancers than wizards. I can even double down on it and say that the subclass should also work for Warlocks, but that's not the point -- the point is there is design space here, and I don't think that space should be closed off. My example could easily have been illusion specialists that were sorcerers or wizards; scirmishers that are rogues or fighters, etc. (I won't even raise possible Paladin/ranger subclasses, but they're there too!)
It would take very little work to adapt the PHB necromancer into something that would be rich and viable for both clerics and wizards. Having that would not stop you from having the necromancer you want. And both could be "really good" -- but they'd be different!