I loved the movie and I liked that the script was a little different than the book. Probably the script writer did not want to make a book adaptation and he wanted to have his own perspective on the story. It is called "to have your own pesrpective". If he were to live 28 years on the island, there would not have been the love story and he would not have any reasons to live and come home. He would have stopped carrying about his life and finally die, but she was his reason to live. That is why the script writer reduced the time he would have returned home, because she was waiting for him to come back.