Whether there was a business case for it or not was a moot point. Boeing looked at it of course and the issue was transonic drag rise. The 747-400 hump ends where the wing begins, so far so good (and actually even better as it smoothed out the area rule curve). If you extend it any further, the cross section curve spikes and drag goes up quite a bit.
The Dreamlifter just doesn't care about drag.
--M