Just because Seju may not end up with Sumin it doesn't mean she'll be forever alone. She's only 34.
If I say Seju would not want to move on with someone else for the rest of her life. Or I say she would try to move on by dating a few hundred random ladies on the street, and after 10 years of doing that, she decides to go back to Sumin again. Or I say that she would fail to overcome her sadness of living alone, so she ends her life at some point. Could you prove me wrong? Positive thinking that everything that goes wrong now would turn right in the future is just another form of building a false hope; it's never a fact.