Cruises charge single supplements.
In the end, customers can find airlines to fly on - it’s up to them whether to pay a quoted price. Supply and demand- if no one takes the seat, the airline will adjust its practice.
The single supplement on cruises is to make up for lost revenue of the miss g passenger who is not buying drinks, specialty dining, spa services, bingo, excursions, watches… Obviously they can’t sell that empty bed, unlike an empty seat on a plane. When my kids travel with friends, they usually book themselves and don’t worry about seating.
Yeah - it talks about only on certain fares, trying to get less price sensitive business travelers.
But just like on cruise - some won’t pay and they’ll match you with a roomie.
Like any pricing model, i’m sure it will be fine tuned based on supply/demand.