This question is what alot of people might ask when they come across my post so ill explain my reasons for liking this rare pair.
My Narrative reasons for liking the pair link to
Firstly Setsuna gets to experience the life Pluto never got to, it sort of builds up her arc more, because in the black moon arc Pluto struggled against the rules and laws of the world for her family. It would create an interesting conflict with the rest of the outer if Setsuna joined them primarily to moniter them.
Secondly its heavily implied that Pluto wanted a family. With it being practically in the text stated that she basically raised chibi usa in the future, and was into neo endy as well as envying the queen.
Thirdly Hotaru gains a semi trustable adult in her life, Serenity knows she needs one. It explains why Usagi would let the outers keep baby Hotaru although THEY TRIED TO KILL HER. [Listen I love Haruka and Michiru but i wouldn't exactly trust them with her after they tried to kill her even if they did redeem themselves the inners and outers interpersonal dynamics would have to improve before that.]
It further humanizes Souichi and adds more depth to his character as well and gives him more of a dynamic with the rest of the senshi and allows us to keep him in the story, as well as further flesh out his dynamic with his daughter.
Also Futher slapstick opportunities with hotaru and kaolinite. Kaolanite having one-sided beef with setsuna.
Also I RUSE TO BELIVE SETSUNA ISNT IN HER LATE THIRTIES SHE WAS WORKING AS AN ASTROPHYSIST IN THE DREA ARC and got a job in stars