If I would not be reading, that time would still pass. I would move closer to death by the exact same amount. Literally regardless of what I would do for 16 hours, my life would move exactly 16 hours closer to death.
The assumption is that if you wouldn't be reading, you could do something else that you would want to do. But if you are reading you can't. If you have more time than you know what to do with, then the argument doesn't hold.
