but in the event of cancellation, no one would be boarded!
Not always. FWIW most of the cancellations that I've been involved in "at work" have happened after the passengers have boarded, so I suspect Paxboy's last observation is correct.
As a passenger I can only think of one cancellation that I had warning about before I'd left for the airport (Mass cancellations, snow, Christmas a few years ago
).