In my own perhaps limited experience, I've never really encountered anyone who is simply incapable of deep thought / critical thinking. I've met plenty of people who have a learned helplessness when it comes to, say, math. I've studied alongside classmates who could't spend as much time studying as me, simply because they had to support themselves with part-time or full-time work just to keep their head above water. I've met plenty of people who are more or less happy with their current job, and spend time thinking critically about their hobby or other non-academic things.
Sure, all other things held equal, maybe inherent intelligence is the limiting factor. But that's the point -- not everything else is equal. There are barriers other than IQ preventing people from achieving more, and I think we'd all be better off helping them overcome those barriers.
Besides, IQ tests are a pretty phony measure of intelligence. They're about as reliable as lie detectors.
Sure, all other things held equal, maybe inherent intelligence is the limiting factor. But that's the point -- not everything else is equal. There are barriers other than IQ preventing people from achieving more, and I think we'd all be better off helping them overcome those barriers.
Besides, IQ tests are a pretty phony measure of intelligence. They're about as reliable as lie detectors.