I could make a lot of comments here but I'd just be reiterating what people much more qualified than I have already said. People who are at the forefront of AI and have actually developed state of the art AI technologies for many years, in some cases decades: Andrew Ng: http://www.wired.com/2015/02/ai-wont-end-world-might-take-jo... "I think it’s a distraction from the conversation about…serious issues," Yoshua Bengi…
Most of these people you cited actually have more nuanced views on the future of AI, e.g., LeCun and Bengio are (along with many others) signatories to the "Research priorities for robust and beneficial artificial intelligence" document (http://futureoflife.org/misc/open_letter, http://futureoflife.org/static/data/documents/research_prior...), which encourages research into AI's social and economic effects as well as research into technical questions of designing provably valid, secure, and controllable AI systems. Unfortunately when the conversation becomes "Stephen Hawking says we shut down AI research now before it dooms humanity", it becomes harder to express the belief that there are potential consequences of increasing machine intelligence that are worth understanding and planning for, even if we are not currently anywhere close to building superhuman general intelligences.
For what it's worth, expert opinion is not unified on AI risk in the way that it is on, say, climate change. First of all, most AI researchers don't spend their days thinking about long-term risks, so their main reaction is "the stuff I'm working on right now is such a toy that it has no chance of ever becoming a threat to humanity", which is probably true, but doesn't really make them an expert on the topic. Of the researchers who actually do think about this stuff, Stuart Russell is at least one example of someone who believes that there are serious questions to be asked (http://www.cs.berkeley.edu/~russell/research/future/, http://edge.org/conversation/the-myth-of-ai#26015):
"None of this proves that AI, or gray goo, or strangelets, will be the end of the world. But there is no need for a proof, just a convincing argument pointing to a more-than-infinitesimal possibility. ... No one in the field is calling for regulation of basic research; given the potential benefits of AI for humanity, that seems both infeasible and misdirected. The right response seems to be to change the goals of the field itself; instead of pure intelligence, we need to build intelligence that is provably aligned with human values."
Hopefully once the popular fearmongering dies down a bit, the field can move towards more of a sane, thoughtful conversation regarding what if anything we should be doing to prepare for the potential arrival of very intelligent (specialized or general) machines in our lives.