I'd actually been thinking that if an ISP offers something along this line it would have a lot of takers without losing anything at all (except expenses for initial changes in software). The idea is that there must be a boost mode (higher speed) and a normal mode (lower speed). One can use the boost mode up to the fup presently being offered and the fup quota is reduced only in this mode. The customer can choose between normal and boost modes at any given time. If the fup is used up one can only use the normal mode (or boost mode could be offered at a price like Airtel smartbytes topup).
With this, the customer can have the high speed at the time he desires, for eg for watching videos or expediting a download that is urgently required. At other times he can choose the normal mode with reduced speed. The ISP will of course have to make changes in their software for bandwidth allocation and usage monitoring for this to happen.
Pretty much what I'm talking about, man. This totally needs to be implemented.