>>8459288I guess it depends on how much life there is outside of Earth. Mori has stated she's responsible for human souls, but is not the only one There's also Reapers assigned to animal souls. There could be a Reaper responsible for ayys or for each planet, who knows? There could even be afterlives unique to each planet because the civilizations have different expectations and faiths. Death-Sensei could be in charge of it all or he could only be responsible for Earth, and other planets have their own beings handling that (and there doesn't need to be overlap so this may as well be the only one.)
Mori is still his first apprentice and that's a position of power and respect.