This is the most important lesson I learned from the masters you mentioned above. They after all, are human too. John had good days and not so good days. Cobham had days where he played amazing and other days, where he played not so amazing. Sometimes both sounded amazing to me, but they were not happy with themselves.
Masters are human beings.
They have the advantage of the higher evolutionary stage, but they are not immune to human suffering. Art gives them a way to digest that suffering in the most fun, creative and noble way, but it does not make them immune to the lures of the Ego. It is great to get a standing ovation and a lot of media attention, but it can also be toxic. When that public attention fades, many musicians take a serious emotional dive. This symptom is worse in the world of popular music because this world places a high premium on temporary fashion and external appearance.