Part of being a manager is to be proactive in trying to retain the good people. Increasing their visibility within the leadership and across the organization, paying keen attention to their career goals, selling these goals in advance to your manager and others within the organization, clearing hurdles that may be affecting their work etc.

And most importantly, plan for their departure. Do you have any internal candidates to take their position? What would it take to hire somebody and bring them to the IC's level?

When they finally decide they want to move on, you have hopefully laid the groundwork to keep them at the company (they move to a different group with an opportunity that matches their ambitions), or you don't have the arduous task of replacing them all of a sudden.