There's always some empty capacity, whether it's non-rev tickets for flight crew and their families which are lower priority than paying customers or people who miss their flights.
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 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.