i once read this fic in cn fandom where chuuya passed on (in the fic) and the comparison they made regarding chuuya and oda in dazai's life was so interesting: odasaku is dazai's sky, and chuuya is dazai's sun.
personally, i think it's somewhat the other way round, though: oda is the sun and chuuya is the sky. in the sense that oda is sth that dazai turns to for guidance and direction, the anchor, the point on a compass, the one thing that might destroy him if he actually took the time to think carefully about their relationship (pls consult this analysis before fighting me). chuuya, on the other hand, is somewhat like the sky: constant, unassuming, you don't think abt the fact that the sky is there. and it's doubly fitting bc the sky itself is a void (in the sense that it opens to the galaxy, so a 'sky' is a thing that is not-quite-a-thing. doubly fitting, if we consider that skk has been apart for more years than they have been together. chuuya is dazai's sky, chuuya is dazai's void, chuuya is the constant that dazai has never lost and will never lose.
in a sense, i think dazai never ever entertained the possibility of losing chuuya. i don't think he ever saw chuuya as something he owned, as much as he jokes that chuuya is his dog (fifteen), we also see how in stormbringer he actively undermines his own words, referring to chuuya as a human etc. etc. it's almost like the 'dog' and 'possession' thing is an inside joke between the two of them, like dazai is saying all that precisely because he knows it will rile chuuya up. but in dazai's own mind i am willing to bet that he's never seen chuuya as a possession; simply because he doesn't want to risk losing chuuya. it's a conflicting perspective, if he doesn't own chuuya, then he can't lose chuuya. so he sees chuuya as like, a constant. that's why he always refers to chuuya as his partner. not ex-partner (as in, partner i have lost). but partner, as in, partner i'll always have.
so if, in the event that dazai does lose chuuya, no matter what the reason, dazai doesn't feel it first. and for some reason i think it would happen when dazai isn't there/isn't aware of the fact that chuuya is dead when he does die (bc there's no way that dazai would willingly let chuuya die, if canon events have been any indication).
and when it happens, he can almost delude himself into thinking that chibi is still there bc his partner has always existed. not exactly by his side, no, but he's there.
then he might, idk, do the stuff he does when chuuya is around. like look for him in a crowd. impulsively decide to break into chuuya's apartment. only to remember, immediately, a split-second later, that chuuya... isn't there.
and i think it hits him the way it hits us when we learn that the sky isn't quite the 'sky', like it's not just a patch of blue weighing the earth down, but it's an endless void that stretches out into the universe, a hole that we can't fill. it's not helplessness. it's just numbness. he doesn't know what to do about it. he can't do anything about it. chuuya likely didn't leave him any last words or wills and testaments; if he did, it would be something stupid. and dazai wouldn't know how to cope.
oda went towards his death willingly: i think dazai could at least make peace with that. he understands it, to a certain extent, bc oda's death can be read as a form of suicide–– he walked towards the inevitable death despite pleas from dazai himself. chuuya's death, intentional or not (on chuuya's part), wouldn't have been anywhere in dazai's plans at all.
at some point he might try to convince himself that chuuya wasn't his to lose (cue: t.swiftie's august), but it won't change the fact that dazai lost him anyway. and that would hurt the most, because now the sky has fallen and the rug has been pulled from underneath his feet and nothing in the world is permanent anymore.
and with all that said, i think, at his worst, dazai might withdraw permanently. just remove himself from all sorts of social interaction. move back to the shipping container and watch and wait as the days pass by until he dies. the ada might try to stop him, of course. but it'll be a long time before he can bring himself to come back.
how do you go back to a world where the sky has fallen?
how do you even begin to comprehend a world without a sky?
how do you begin to wrap your head around the fact that the sky has always been a void, and that actually, even if the sky falls, nothing will have changed?
it's maddening, and dazai will hate every moment of this existence. even more so than he did before.
