Good question. But i dont think anyone knowns the exact ans.
Well a time will come when your pro stats will decrease gradually with his age. Your pro wont be able to complete the objective. He will get injured frequently and will miss first team action. Then he will be transfer listed. You have sign a contract with a small team. There also you will miss games due to injury. Finally you will feel fuckd up, and retire and then continue your career as manager.