That would make Mercantilism the best civic in the game. Nothing does +20% commerce and for a reason. Mercantilism is fine where it is - a sometimes civic for the civilization without any friends that gets overshadowed by Agrarian. The solution to Agrarian shouldn't involve creating a second overpowered civic in the same column. Civics that see less use are acceptable. Its rare that you'll use Pacifism, Liberty or Slavery unless you're doing something very specific, but they've got a place.