The biggest threat isn't the AI itself
but our ability to think being eroded.
"computer says no" will become normal and accepted. there are already examples of people following dumb devices like sat-navs onto railways, off cliffs, into fields etc.
AI is not creative, does not handle new situations well but we will all trust it. Suddenly all data about behaviour will be based on machine learning too, thus ensuring it will tell you what you want to hear, rather than what is going on.
AI in itself can be useful, but the danger is that it is assumed to be capable of being rational, and having (increasingly un-)common sense..