Why is there always a vacany for a Finance Director?

If the FD is really good, he will be headhunted - leaving a vacancy 

If the FD is really bad, he will be fired - leaving a vacancy 

If the company prospers the FD may well land the top job - leaving a vacancy 

If the MD dies or retires only the FD has sufficient familiarity with all aspects of the business to take over - leaving a vacancy 

If the company is doing badly, the FD will know first and will leave before anybody else finds out - leaving a vacancy 

 (According to Peter Vaines and Roger Nuttall in their book "The Bottom Line")