Realistically this is the best outcome. He's old. She's young. They could've gotten together than the fighting and bickering would've started after the honeymoon period. Then there would be a bunch of arguing finally with some messy breakup. She would move on. He'd be even more depressed that he let it get to that point.
It sounds good on paper them getting together. But in reality it would've sucked. This is the ideal ending. She will find someone closer to her own age. He will most likely work himself to death lol (alone).
Not a great ending but not a bad one as well. Thought we'll get some closure regarding Kousuke and Miya relationship. Is she still a brat? But still at least Kousuke is not as pathetic as he was in the beginning. He finally learned to sy fuck it and ignore what society says. Do what you want as long as no one is harmed.