Chipotle recently promoted COO Scott Boatwright to the CEO position.The departure of CEO Brian Niccol has brought uncertainty to the investment case for Chipotle (CMG -0.41%) stock. Niccol was the company's most successful CEO since founder Steve Ellis stepped down, and his process improvements and innovations helped Chipotle stock rise by around 775% during his six-year tenure at the company.Now, with Niccol's sudden departure for the Starbucks job, investors are right to wonder what is next for the compan ...