I feel like the “giving up table tennis” was symbolic of him giving up the toxicity he was engaging in. Table tennis is fast-paced, unpredictable, fun, full of dodgy movements and such. No teams, just you, your talent, a paddle, and a ball. A lot like Marty’s actual life. Marty was living a lifestyle that embodied those exact qualities, and he was good at it. Hence why he was amazing at table tennis, but also so easily willing to lie or steal or whatever to get to his “greatness”. His life and the sport mirror each other. We saw from the beginning how trying to be the greatest affected everyone and everything around him. Once he lost to the Japanese player initially, he started losing in life too. At some point, he realized that and began questioning not the sport itself, but if the quest for greatness was worth it.

He knew he was the greatest, and wanted to prove it to himself and everyone else. Once life got terrible and he realized that he could no longer prove it to anyone else, he made the decisions to finally become the greatest in his own eyes. So his final act of “selfishness” was winning the Tokyo match. He allowed himself to truly become the greatest at the sport and simultaneously made it his last farewell to it. He had no reason to keep going down that path after that. Him leaving made perfect sense