I think this is a great idea. Similarly ISPs should detect if they don't receive any packets from your connections for any 24 hour period, and refund you for that day.
Well, customers who take holidays would have days when they don't transfer any data (assuming they turn off their router/modem while they're away) and customers whose connection is broken because of faulty ISP equipment would also benefit (because there would be financial incentive for the ISPs to fix the problem, rather than it helping their bottom line).
Still doesn't really make sense because the ISP still incurs the same expense. They had to buy that capacity which still exists while you are taking a short holiday. If its a long holiday you can cancel the service for that time.
If your service doesn't work for a significant period, you likely can receive some compensation enforced by a government body.
Not even. During the process and confusion caused by changing the pricing rubric, they'll make sure that you're paying more than you were paying before.