I think the worry is that at some point someone will crack a self-improving general AI and at that point the time between "we did it" and "oh shit" could be on a human timescale very very short.
Given the potential downsides are possibly extreme (however unlikely) I'm happy that people are paying attention.
I think a lot of "AI" is hype and hyperbole and largely those people don't worry me that much, its the quiet ones doing fundamental research that worry me a bit more.
You are talking about a breakthrough with a threat level in the same magnitude as nuclear weapons, dinosaur killing asteroids etc.
Personally if I had to put my money down I'd say I wouldn't see a human-level general intelligence in my life time but a self-improving/learning intelligence doesn't have to be human level to be very very dangerous either.