Well, before we all get too maudlin here and start crying in our beer,
leave us all please to remember that there IS an upside when a kid moves out:
You did your job.
You’re a success as a parent, the biggest success there is. You equipped your offspring with the will, and the mental toolkit, to leave the nest, like he’s supposed to, and no amount of successfully staged birthday parties or trips to Disney World or shopping coups at the mall can ever hope to stack up against that. Go, you. 
And there is another, lesser, upside: you get a room back. Bonzo’s room is currently going rapidly back into “Mom’s Craft Stuff” configuration, where it will presumably stay. Oh, his bed and dresser and shelves with Spiderman and Legos are all still there, but I get to spread my craft stuff out all over the floor and the bed, instead of having to tote it downstairs to the dining room table if I wanna do something.
And I no longer have to contend with the piles and piles of D &D paraphernalia all over the living room. And I no longer have to contend with having to wade through hip-deep piles of junque in order to get at the windows in his room.
Those are all his landlady’s problem now. 
And it’s one less person to need a car–three people divided by two cars is much easier math than four people divided by two cars…
And I no longer need to keep buying lunch meat and beef stick which offendeth mightily his vegetarian sister…
And no, I am not getting ready to go, “WAHH! I wish I had those problems back! If only I had him back,” because truly, kids are meant to move out. Think of the dysfunctionality of the boy his age next door to us, who lives with his grandparents (his mom having died unexpectedly of a brain aneurysm last year, otherwise he’d still be living with his mom); who just barely made it through high school; who works in a cafeteria; who “might take some classes at Richland”, but who we all know won’t. If that were my son, I’d be stricken with guilt, “What did I do wrong, that he won’t leave?”