Ossified professor syndrome

During the lecture series offered by an Extremely Famous professor some while back, a number of us at Aberdeen got to talking. Why was it that by the time an academic had become the invited star at a famous lecture series they had absolutely nothing new to say?

Theories included:

1) By the time you’ve got to that stage you’ve got nothing new left to say. This is a version of the ‘all mathematicians do their good work before they are 25’ theory.

2) The Committees That Be would never have the courage to invite someone exciting to give a lecture. The more paranoid version of this is the suspicion that only the toothless are put forward for high profile public roles. Even Zizek, much as I enjoy his snarls, is clearly a pet beast.