Mastery is a peculiar notion. The "master class" format is a staple of classical music training. What does it imply?
For one thing it is probably more aptly called "fame and fortune class" as the level of mastery of the "master" must always be weighed in relation to a complex set of questions if not answers.
When Karl Ulrich Schnabel (son of Artur) gave a master class it was truly a class given by a master teacher. As such the audience consisted of as many psychologists as pianists. He knew how to address all manner of student with candor yet without condescension. I recall specific instances of these classes with deep admiration.
Yet even he was confused, it seems to me, about his own musical mastery. How appropriate is it to inflict one generation's notion of mastery on another generation for whom that notion may be totally irrelevant?
I had an argument with him once about precisely that point: As I listen to music, it seemed to me that some of his young private students played like old men. How can that happen if not that interpretation has been locked up somewhere, to paraphrase Emily Dickinson, in a room "the sexton keeps the key to."
