No Toyota leader has ever assumed that lean sustains itself. On the contrary, the company's leaders intuitively understand that the natural evolution of a culture will atrophy and deteriorate unless its leaders are continually renewing and passing the DNA to others.
Only by adding new energy... can you maintain or improve a system. Leaders are the primary source of energy; they arrest the atrophy of lean tools and keep them thriving and evolving in the culture. It is they who can sustain lean thinking.
From the Book The Toyota Product Development System: Integrating People, Process and Technology, Productivity Press, 2006, ISBN 1563272822