TightSlot,
The sort of airline likely to follow my hypothesis is likely to be a small airline, with limited routes, operating from small airports. I agree that the majors wouldn't, but a small carrier in say the US mid west, flying between not very big places, could certainly find that approach attractive.
If the 9am out and 10 am return are so lightly loaded, and by 8 am they would have a very good idea if the loading on the 3pm out and 4pm return made it possible, then the cancellation would be much more attractive.
Just because most airlines won't do it for all the reasons stated, it doesn't mean that it never can happen. Just as most airlines would not deliberately not load baggage, but some of the small aircraft in the US can get to the point where, with a full load of PAX, they can't stand the weight of all the baggage. It happened to me from Cedar Rapids to Kansas City once - and then they had a devil of a problem because it hadn't arrived when I left the hotel the next morning and it chased me to Chicago, Fort Wayne, back to Chicago and on to Fort Lauderdale before catching up with me two days later!