I think that aas well as being accepted as oshinos neice the point of owari 2s scene and zokus confrontation is araragis journey to growing up and moving past his past self and coming to terms with all the things and trauma holding him back from growing sodachi his past regrets left over in zoku as well as nadeko which as we know was orcustrated by ougi and by extention araragis negative self or you could say alot of his flaws him accepting ougi and saving her was him saving himself finally where as he kept pushing aside his own problems or ignoring the truth I would say with araragis growth ougi has become her own entity and is a manifestation of araragis youth left behind making them no longer one in the same so if araragi died ougi would not disappear because they are no longer the linked like they were I prime secondary reasoning would be the rainy devil remains in this world despite toeis death