The senior personnel office in the Privy Council Office worries a lot about it. They have in mind someone who would replace someone. But as Mr. Desautels says, it's difficult for a deputy to be grooming a successor within the department.
I'll just add this to underline the perception from outside different kinds of institutions. In the university world I work in now, the standard term is five years, automatically renewable to ten if you're doing a reasonable job or an excellent job, like all the presidents in Ontario do. It's just unheard of to think of people leading major organizations on a turnover of less than three years.