The fact that Satoko sometimes imagines herself with Mashuu's same age means to me that what she has to settle is not her relationship with him, but inside herself. Well, here we talked about what it means to be an adult, an answer may be that, even if they are assholes, she cannot be influenced so much by what her parents do or say. In this field Mashuu instead seems to have grown a lot since he seems to have learned to live together and accept his father's defects and humanity. The theme is no longer "Satoko must save Mashuu" but "Satoko must save herself", perhaps having Mashuu as an example to follow.
For this reason I am not very excited by the idea that the two end up together, for their "salvation" they must find their way separately, if they think that the solution of their problems lies in the other person then their problems will only remain closed in a drawer and they will get stuck in their path.