^I guess a "mourning period" works when a member is retiring or their future is completely open, like most of the recent departures (Sakichy, Eri, Junjun, Linlin)... That is, their thread doesn't need to be moved instantly to grad plaza, because they're not instantly moving on to their next public career.
But with Aichan, we already know she has *something* lined up (..that play thing that I don't remember anything about haha) so there's no need to linger!
I still hold on to the right of having a "mourning period" in the future for graduations, depending on the situation ^_^