I think Duchamp would have had a ton of fun with the internet. I think Dali would have found a way to make even more money with ridiculous stunts and frauds. Not sure about Magritte. I have seen a few exhibits of his, and I think he creates fantastic imagery, but I can't profess to know too much about him and his motivation. I think Banksy read the Fluxus Manifesto (pictured below) and thought to himself, "Well these guy were right, but why did that have to create such shitty art?" So he decided to do something about it.
Haha, it often seems like those that write manifestos end up creating shitty art, or art that doesn't really appeal to many people. Ideas and execution need balance. That balance is struck through feel and feedback. It's tough to get good feedback when you've issued a manifesto that is likely to provoke strong opinions about what you may yet create.