Presumably they depends on the exact specifics of the economics involved - the airlines are only incentivised the way you say if there's more money to be made f

Presumably they depends on the exact specifics of the economics involved - the airlines are only incentivised the way you say if there's more money to be made from the amount of revenue brought in by people paying to jump the queue than savings to be had from the fact that for every minute less their typical boarding takes is a minute less of time they need to have a departure lounge allocated for each plane, & a minute less of time for a number of staff working it.