I don't think the dining plan was under-priced as much as Disney didn't anticipate how guests would use/abuse the plan. I doubt Disney anticipated the number of guests who wouldn't "waste" a child credit on an child meal. I don't think Disney anticipated the number of guests who would stretch their credits, thereby reducing restaurant capacity, by sharing meals. I don't think Disney anticipated the extent guests would be looking to "max" out the plan by gravitating to ordering the most expensive menu item and the most expensive restaurants.
I think the dining plan was fairly priced, based on how guest normally order food but not based on the ways guests were able to "game" the plan.
A previous meal plan became something like pay $20 and get $25 worth of food.