Wolfe's law
It states that continual use builds your body, but overuse breaks it down. So why build. Why train the body. Deterioration of the body is natural. There is nothing you can do about it. What you can do, is you can help keep it strong and useful longer.
Something I have been thinking about for the past year is "unbalanced training" I like to call it.
Never doing the same exercises more than 1 day a week. Constant changing will keep the body fit and minimise damage and overtraining. Your body will have to adapt to these constant changes which will have positive benefits to you.
The fact is your body can't be at it's best of worst for any extended period. If it does you will face negative feedback such as illness and other issues. What I propose is a constant fluctuating fitness routine moving in and out of different fitness levels.
Lets say fitness is a line like the one bellow (simplistic example).
----------------unfit----------------fit---------------------Athlete
--------------unheallthy----------healthy------------Athlete
What will happen is you will move from different levels of fitness and health creating a zigzag or wave effect that I think is the key to remaining healthy and fit for extended periods of time. Plus it allows for periods of inactivity and eating various foods. This is just a small portion of the system and how it works.