I had a cancelled flight recently and they booked people two weeks out because every flight from that day onward was full or nearly full. I showed up the next morning and was able to board the next flight because exactly one person had scanned in their boarding pass (was present at the airport) but did not show up for whatever reason to the airplane.
Beyond that, people just make alternate plans, whether it's taking a bus or taxi home, traveling elsewhere, picking another airline, anything is possible.
I work in logistics for a FMCG company and sometimes our main producer goes down and we run out of certain types of stock. We send as much out as we can and cancel the rest.
If they really want the stock the customers can rebook an order for tomorrow because they aren't getting it today. And we just start adding extra stock to each delivery.
It's the best of a bad situation.
We don't have the money to have extra trucks and very perishable stock laying about and I know the airlines don't pay 300 grand a month to lease a 737 just to have it sat about doing nothing. There's very little slack.