100 calories of nuts or broccoli costs a whole lot more than 100 calories of bread or rice, and this is reflective of the greater agricultural demands of pretty much everything else versus bread and rice.

Even rice is a substantial improvement in glycemic index over refined wheat and sugars. If it does turn out that empty carbs are a major public health problem, there are incremental improvements that can be made all over the world regardless of poverty.

Nuts are hardly our saviors. Most nuts are expensive & environmentally unfriendly. Which is not to say you shouldn't eat some nuts, but they can hardly step in as a global staple.