You definitely need a very relatable character for this, but I don’t see how Melinda Dillon couldn’t have served as a blank slate. She has some characteristics that make her unique but not enough to make her unrelatable. I definitely related to her and didn’t at all to the lead. Maybe that’s just me but I don’t see how her character would exclude anybody from projecting themselves onto her.
The issue isn’t Dreyfuss vs. Dillon, it’s that if the film is all about finding her son, it’s suddenly a very different film altogether.