Also dropping all the way to 10kbps with enough use would just suck. It's effectively unusable and it would be extreme penny-pinching to make sure the maximum 24/7 user can't squeak out more than 3GB extra on their 100GB plan. You get more variance than that from different month lengths.
Bandwidth is use-it-or-lose-it. If nobody else was using it, then it doesn't hurt anything. And during high demand traffic shaping hopefully gives their traffic even lower priority.
On networks I manage, there are clients who pay for large quantities of super low priority capacity - eg. for moving scientific data around, or backing up stuff that only needs to complete sometime in the next 30 days.
That means there is no such thing as unused bandwidth - almost every link is 100% full of paying customers data, and anyone using more displaces one of those low priority customers.
A better limit I think is to limit the user to 10 kbps over a rolling 24h window, 100 kbps over a rolling 1h window, 1Mbits over a rolling 1 minute window, and 10 Mbits over a 1 second window. That way they can quickly check an email or load a web page... But it quickly slows down if they try to (ab)use it for hours on end.