You have just assumed that we need a purpose when that's far from the truth. We humans don't have the need to be "useful". We have the need to be "competent". And that will be through hobbies. No, the world won't become like the one you saw in wall e, instead it would be a world where everyone is busy learning skills for the sake of it. Ye sure you could argue that machines could do it better, but humans don't give a fuck. We like to compete and flex on each other. Chess for example, it has already been infiltrated by AI, but that doesn't stop chess players from playing. Hell, they are having more fun than ever now because of the weird bots they can use. What I'm saying is that humans will always want to flex one another. That sounds like a bad thing, but it's fantastic actually. Arts and sports will never die, because we will be too busy using them to compete with each other..