Also, take note of the fact that it wasn't Mokou who did Kaguya in.
Poor Kaguya, she's the type of immortal that can die but never really stay dead. Technically speaking she'd be the perfect test subject for Eirin...if she wasn't a princess.
Torisuna said: Maybe she's gone insane from carting Kaguya and Mokou all the time.
Day in and day out, carrying the same two dead people. And yet there's always another to take their place. Even with Youkai regeneration (see Mystia+Yuyu) that's a bit excessive.