If they had left, it would have been a different novel altogether, and we would not have the deeper understanding of what it was like for Iranians to live under this regime as the years went by. A big part of the author’s expressed purpose for writing this novel! So I have a problem with this question.
I certainly believe the author developed these characters well enough that we can understand their reasons for staying. Both of them: Amineh demonstrated her attachment when she chose to remain at home with young Sara, to stay with Jalalod-Din and Manuchehr, during the night in 1985 that their neighborhood was targeted by Iraqi bombs. She would not have wanted to emigrate any more than Farzad did—-and certainly not without him. Loyalty is a defining character trait of Amineh’s. And despite the challenges and disappointments of her marriage, she does love Farzad and knows she too bears responsibility for their relationship.
So yes, they made the decision that was right for them.