It's not really about complexity. The restriction is the ability of humans to sit on their butt in a room and determine every possible requirement. This doesn't

It's not really about complexity. The restriction is the ability of humans to sit on their butt in a room and determine every possible requirement. This doesn't work. As soon as you start development you learn. As soon as you interact with the environment you learn. As soon as you interact with customers you learn. You never stop learning during the life time of a project which means requirements are never complete and can never be fixed in stone.