That seems to require a lot of additional tracking for the player or DM and doesn’t allow to many spells to be cast using Grey Casting before having to drop back to plant energy without becoming undead. Did you have a particular type of undeath planned for this or a newly created kind?
I tried to resolve this and the other alternate power sources here:
For Necromants it was 1 extra roll per encounter and potentially an extra one per spell. The only other tracking was an additional encounter when leveling against at low levels, an admittedly more powerful creature than yourself. That creature and the potential Grey Zombie attacks were what kept more Necromants from existing.
I feel your version as is turns wizards into a few “free” casts per day using an alternate source and maybe even rewarding them with undeath. Is there something I’m missing?
Your question of “Why are the Sorcerer Monarchs not necromants?” can be answered with “It’s not more powerful than just defiling”. My own version’s answer is “It’s not as powerful, reliable enough and it’s dangerous”. Defilers don’t care about the damage they do, they do it in exchange for speed and power (feats) and only care about themselves. Preservers would probably like your version of necromants though, so “Why aren’t Avangions necromants?” becomes a good question.