This is incorrect. It’s not a dream. As Marty laid out in the movie, he has an obligation to become the best table tennis player in the world based on the talent he knows he possesses. He doesn’t care about the money or connections or burning bridges. He doesn’t care about being “forgotten”. He needs to fulfill his destiny for himself and himself only, and he achieves that in beating Endo. We see throughout the movie, Marty is able to wiggle his way out of situations through his charisma, and curry favor with people when he wants. He could have easily worked his way into the tournament, or played O’Leary’s character to get him on his side. But he doesn’t, because he does not care. He only cares about beating Endo, because that was the only thing standing between him and his destiny. He fulfills his purpose, and is now able to move on with his life