It, really, really wasn’t.
Calia was resurrected by the light. Performed by a freakin’ Naaru.
The Forsaken were resurrected by death aspected magic.
I’m not against it. It just needs to follow the rules established in the lore. A new faction of undead would be a more accurate situation, led by Calia. They could welcome the Forsaken into their ranks (as she would) but they would need to be lightforged in some way to wield it. Maybe a second death, and resurrected by the light.
The thing is, anything is possible in the game - Zandalari druids are pretty freakin’ neat but they never would have made sense before the lore was written in a way that allowed them.
Blizzard just need to write the universe in a way that allows things, which means literally anything anyone wants could be possible, but they like to pick and chose based on arbitrary things.