It might depend on the airline. I used to travel with Ryanair frequently, and special tickets (whatever they were called) were only available for 1/3 of the plane’s capacity on a first-come-first-serve basis. Those upgrades got you to choose your seat, skip the queue and guaranteed space for a carry-on bag. All of those things follow a similar pattern: if everyone did it the system would break, which is likely why they picked 1/3 as a cap. It’s actually quite clever, although I still dislike the ongoing enshittification of air travel that the budget airlines have caused, despite benefiting from it for a couple of years.
It might depend on the airline. I used to travel with Ryanair frequently, and special tickets (whatever they were called) were only available for 1/3 of the plane’s capacity on a first-come-first-serve basis. Those upgrades got you to choose your seat, skip the queue and guaranteed space for a carry-on bag. All of those things follow a similar pattern: if everyone did it the system would break, which is likely why they picked 1/3 as a cap. It’s actually quite clever, although I still dislike the ongoing enshittification of air travel that the budget airlines have caused, despite benefiting from it for a couple of years.