That's what Chipotle Mexican Grill ( CMG -1.87% ) decided on Monday after more than a year of plummeting sales following 2015's food safety crises. It was about time for a head to roll as the stock's 50% drop and 10 months of comparable sales falling 20% or more called for someone to get axed. On Monday, Chipotle Founder Steve Ells announced that his co-CEO Monty Moran would resign his post and his board seat effective immediately, and retire in 2017, making Ells the sole CEO.