As you get older, you will have less time and less energy. As you age, your productivity may continue to increase even as some parts of your body and mind decline due to the wisdom of experience, but sooner or later it will level off and decline. It is a sure thing, though the exact time varies due to factors within and outside of your control.
Whether through old age or accident, and whether permanently or forever, your capacity is going to wane. When it does, the life plan will help you feel greater satisfaction with what you can do no matter how much or how little that might be. There is no better way to prepare for the eventuality of a reduced output than recognizing and learning to cope with the fact that there is more than you could ever do even while you are in your prime.
As you grow comfortable contending with a never-ending to do list now, you will be able to instantly adjust to any decrease in capacity when the time comes, because you will already have adopted the system that enables you to feel fully satisfied and joyful in spite of the fact that you can never do everything you imagine doing. This is as smooth as fasting when you are already on a no-carb diet: you can instantly begin metabolizing stored fat without the prolonged discomfort of switching over from carbohydrates.