An open area is not a poorly designed area. It would be boring if the map were a clusterfuck of enemies, chests, and puzzles. There's little reward in exploration if you find something every 5 feet. The open areas are aesthetically pleasing, let you freely navigate with your horse, give you room to fight enemies and bosses, and reward the player for finding secrets and treasure.
2D Zeldas don't need open areas because fields are boring to look at with an overhead perspective, and it would be difficult to figure out where you are because you can only see a small rectangular portion at any time.
Past 3D Zeldas have failed with open areas because they are too far separated from the more linear parts. OoT, TP, and SS all had big boring areas that only serves as a hub for the actual interesting areas. Wind Waker only had problems because sailing could get really tedious, but stuff like the Quick Sail from WWHD really helped solve that issue.
Breath of the Wild takes much more inspiration from the 2D Zelda games like LTTP or Zelda 1, where there is no hub that separates all the areas. The only difference is that Breath of the Wild is much more spread out, but based on the Great Plateau, I don't think that the dev team will have trouble filling the world.