This is the key issue - when is it not worth it anymore? If you are a fan of the University, and view the football program as something that serves the needs of the larger University (in other words, actual USF fans), we're at that point where the return on investment is questionable, and it might be time to shift those resources elsewhere.
If you view the University as existing to serve the needs of the football program, then sure, keep throwing money down the hole. But don't pretend you're a USF fan.