Stephen Hawking is an optimist. He always says that a hyper-intelligent AI would be disastrous. But what's the alternative? The way we're going, we're about to devise our own extinction via global warming, nuclear holocaust, various Malthusian catastrophes, etc. sooner or later. Being replaced by a highly intelligent AI would actually be better, in that there will be some sort of legacy from humankind that will live on. We might actually get lucky and be able to survive as pets, although a smart enough AI will probably not care about pets.
We're all gonna die anyway, so who cares if we die of mundane causes or we die to give way to a superior intelligence? Given a choice I'd prefer for (actual) intelligence to go on.
We need a right good old fashioned war or pestilence otherwise the world will become overcrowded
Plus, wars create jobs just like race tracks help the glue industry