I agree - Forcing someone to stay when they want to move on is pointless. Most people would start looking elsewhere straight away.
The best thing your manager can do is reach an agreement where you stay in your current role (full time or part time) for a period of time while a replacement is sought
The best thing your manager can do is reach an agreement where you stay in your current role (full time or part time) for a period of time while a replacement is sought