I believe it's the top management of the organization, which, in consultation with HR, should formulate an effective career planning strategy to ensure that talented professionals do not leave the organization mainly due to the lack of promising career opportunities. It's the responsibility of the high-ups...
A good career plan often succeeds in retaining good employees. Learning organizations take this aspect very seriously and believe in Human Resources, not just machines. They think it's the people, not machines, who run the systems and transform the world. Keeping this in view, they offer very promising career paths and succeed in retaining the best people most of the time.