I don't see how Shikura would be a good ending, but Fumi route is also nuked, soo.... I think he will end up single, because the logic in NTR stories is that good people deserve to suffeer while scum's will live free of guilt. Fumi will follow Sakura's mentality and won't have a a proper discussion with Shinobu... In the end people should've expected that. Good man won't get a happy end in NTR dramas, never. Unless we're talikng about the isekai/fantasy genre, So... the story is rather realistic in that regard... what is not so realistic is the MC being so naive affter all that, but we know that he's cursed by the NTR devils, so he can not mke wise decisions.
MY opnion is: Maybe the main couple were not meant to be, simple because their enviroment is toxic and one of theen is naive end the other is insecure and prone to cheat (everyone knew, the sings were there)
Eithe way, the author choosed ego over the dignity of her own work. I wonder... some day we will be saying "Plese don't Grapara us" to some new manga with similiar plot.