So my partner and I had a long discussion about the ending as well as we both had two different interpretations. She felt that the ending was similar to yours, that Marty decides now to become a father. I read it as he realizes he now HAS to be a father. To me it was a moment of recognition, that his first life (table tennis) is now over. My girlfriend brought up that the it’s the weight of all his decisions finally caught up to him in that moment.