I've taken courses in primitive wilderness survival, and one of the staple foods was grass seed.
Also lots of roots are edible with cooking, and it looks like we've been cooking for about a million years. Then there's wild rice, cattails, beans, berries, all sorts of stuff.
I agree that most wild plants are high in fiber and low in sugar, but there are are a lot of complex carbs to be had, if you have fire.