In Mozilla's case especially, such a gamble ("gamble"—I really don't think it's that out-there) seems eminently worth it, since they've been treading water at best for over a decade. What's the worst that can happen? They fail? Already happening.
I suspect this entire trend (across the economy, not just at Mozilla) is due to some combo of our modern intense aversion for taking responsibility for anything whatsoever, and pervasive self-dealing in the management class. Board won't replace the "right" kind of person with the "wrong" kind because it's Simply Not Done—why, if anyone in the top half of the intelligence bell curve and an OK work ethic could do their jobs with just a little experience and training, they might no longer command such insane salaries! Can't have that.
But, I'm open to the possibility that it did in fact become unworkable, for some reason I don't know about, to just train people into these positions as-needed, and in fact we do have to pay stupid amounts of money to a tiny, incestuous c-suite class who are the only possible candidates for these roles, or else everything will fall apart.