As they (Smoky and co.) pointed out, McQueen is old. Yes, I agree that it seems wasteful to have given up in the middle of the race after all of the training he went through in preparation, but it was also a HUGE CHARACTER DEVELOPMENT. Him becoming Cruz's pit crew chief was, in a sense, a final nod to his mentor Doc Hudson. It was the perfect ending to the trilogy, and it's slightly irritating to see a bunch of adults tearing apart a movie that was primarily aimed at families with children simply because they were butt hurt that it ending with him moving on to a new chapter in his life.