Our body is breaking down constantly, but it's also rebuilding itself. It's not an inanimate object which wears down until it breaks.

I view exercise as giving your body, and brain, proof that the rebuilding is still needed, and information on where to direct resources.

If you never take a walk, why expect your body to rebuild itself to be good at walking? Eventually it'll lose its capacity because it (correctly) "learns" that isn't important to you.