Dumb agent could also cause human extinction. "To kill all humans" is computationly simpler task than to create superintelligence. And it may be simplier by many orders of magnitude.
I seriously doubt that. Plenty of humans want to kill everyone (or, at least, large groups of people). Very few succeed. These agents would be a good deal less capable.
At some point soon, I'm going to attempt to steelman the position of those who reject the AI risk thesis, to see if it can be made solid. Here, I'm just asking if people can link to the most convincing arguments they've found against AI risk.
EDIT: Thanks for all the contribution! Keep them coming...