I think for humans, the limit of stupidity is extinction. How close we are to _that_ limit, I don't know. Maybe we exist for centuries or millennia, maybe we die out a decade from now. Either way, I'm not too stressed about it
Lacking any evidence, I have to choose to believe that that stupidity of America as a whole has limits. I'm just not sure how close to those limits we are, and that scares me.