Let's not make the mistake of assuming that these two explanations cover all cases of high ranking teachers who no longer post. Likewise, let's not make the mistake of assuming that anytime someone refuses a challenge to "put up", it's because they don't have the goods. Unfortunately, it seems like this assumption is all too often made in these debates. There are many reasons why someone might choose to not "come out and play"; fear/expectation of failure is only one of them.
That's true, but it starts with their making a claim that stimulates a question.
No claim, no blame (to paraphrase Lao Tzu).