It’s not too hard to see that airlines let people go, offered early retirement and buyouts for people. This was from the simple gate agent and baggage handler to pilot and it all has an effect. The biggest being pilots where usually you would have ample on call pilots for people being sick, or standby in case a flight is delayed at a hub in which the route would be filled by a different pilot at the airport. This just doesn’t exist or is is at a minimum currently. Throw in that pilots need x number of hours and simulator training after being furloughed and it’s a staffing disaster. So what has to happen, the number of flights need to be reduced, along with pent up demand the ones that do fly are absolutely packed and if something does happen there’s no longer a margin of an empty flight leaving late at night etc…
Basically all airlines are operating not by choice the Spirit model.