wither

To wither is to shrivel up, to become dry, or to weaken and droop. (verb)

When you become old and sickly and unable to walk, this is an example of a situation where you wither away.
When a plant isn't watered and it dries up, this is an example of wither.
When interest in a social program wanes and it begins to stop being funded and operated, this is an example of a situation where the program withers away.

See wither in Webster's New World College Dictionary