Fully refundable tickets create some of the challenges that lead to overbooking resulting in a ticketed passenger with a boarding pass getting bump down in seat class or back into the terminal as their "plane" backs away from the gate. Not much fun when need to get somewhere on a tight schedule like tucking the kids into bed at the end of the work week or making a connection at the next airport.
Can argue the incremental price increase of a fully-refundable ticket should make the airline whole if some of those passengers bail but even if the airline has the perfect algorithm to end up with exactly the correct number of ticketed passengers as seats on the plane as anticipate correctly re no-show sick passengers or got stuck getting to the airport and through Security or are on an inbound connection that is now delayed.
All of that muddies the water of precision balancing of tickets and seats. But wait, there is more to balance.
A Federal Air Marshall requests (not really a "request", though) a seat or 4 with that notification typically 24 hours prior to takeoff.
A Federal Marshall needs a seat or two to three to transport a prisoner.
An off-duty pilot needs a ride and their union contract says the pilot will get a ride in first class and other flight personnel get economy seats on their way to duty at another airport.
A seat cannot be made air-worthy in the turnaround between landing and takeoff so capacity is reduced. Yep, vomit and mechanical failures happen.
Or, the dreaded mechanical issue takes a plane out of the service and the replacement is a different capacity. This happened to me and Jamie Lee Curtis (she was not quite 30, years prior to True Lies) when she got bumped with her infant adopted daughter and nurse/nanny from First Class so had to sit with me back in Business Class on a NYC-L.A. Friday night flight in early 1987. My flight was memorable. Hers was miserable as Business Class seats were not "lie flat" back then.
So, even if the airline would only sell tickets until exhaust seat capacity then someone with a paid ticket and boarding pass can still have a bad day after involuntarily being bumped down in seat class or entirely off the plane.
I used to fly out of Dallas from the late afternoon to early evening on Fridays. I might book seats on two flights and both fully refundable on my employer's dime. You could do that then which was early 1980s. Maybe still can.
Executives wanting to get home in time to tuck the kids into bed at the end of the work week were frantic.
Was like shooting fish in a barrel as these Friday flights were always oversold by American Airlines. I would walk up to the gate and let the lead agent know I was willing to get bumped if the need arose. I was on a first name basis.
So, I would get some $$$ for the weekend fun plus a free ticket to anywhere American Airlines flew in the Lower 48. I never ended up stranded overnight and I had no urgency to get home. Was a game so I played it.
I began catching rides on 727 metal in mid-1960s when was not uncommon for men to wear suits and women to wear dresses. Smoking was allowed and ashtrays were part of the arm rest. Flight attendants were more cocktail waitresses than safety officers. Simply asking the stewardess for a deck of playing cards would get you a deck for free. Booze drinks were paid for with cash on the plane. You got a full can of soda with a glass of ice plus a second can was freely offered. Your boarding pass was a computer punch card then later was a 3-part typed or hand-written ticket on onion paper. Was not uncommon to have two passengers with the same seat assignment and in those cases generally who got to the seat first kept the seat. Fun times.
