to be honest, and I know this is an unpopular opinion, but toujou and manaka had a kind of parasitic idealistic relationship with each other, where they didn't really understand each other at all.

toujou didn't mind all of manaka's shit and just blindly idolized him

tsukasa had actual standards, man. she wanted manaka to grow, she wasn't willing to accept his unassertive bullshit.

the problem is that the ultimate flaw of manaka is that he's an undecisive child who doesn't understand his own feelings. tsukasa saw the man in him and wanted him to grow to be that man. every time he disappointed in this aspect, she was fed up with his shit and left.

toujou on the other hand had no understanding of manaka as a person. she saw him as a perfect being and didn't really mind or arguably even noticed that his heart was in like four different places and he treated all the women in his life like shit.

my point is that nishino and manaka are right for each other because they can make each other grow and flourish. toujou's expectations of manaka are stagnant, they would not benefit from being together.