Why having a “Vision” alone is not sufficient?

Iconic companies have been driven by a vision but that alone has never been sufficient.

There is nothing wrong in pursuing a VISION for greatness.Where iconic companies like SouthWest , Apple , Costco stand out from their peers is that these companies have continually refined the path to greatness by being willing and flexible to FACE BRUTAL FACTS of REALITY.

This has been scientifically explained by Jim Collins in his book titled Good To Great.