I guess the ending was left that way was because they had a kid, they were always gonna be in each others lives so there's no point in giving the audience a clear cut answer on what decision they decided on. Gosling has said that it's just a story(the present) about 2 bad days a couple are going through, so I like to think they end up staying together. These things are too common and a very well made movie on how relationships get tiresome and people lose interest. Good stuff.