Full service airlines can try low cost fares with restricted economy tickets to fill up the back end. I recently enjoyed a weekend away on a legacy airline for a comparable fare to a low cost once baggage and extras were added in.
The ticket was non refundable, non changeable, accrued fewer miles, had min/max stay requirements and the only seats that could be reserved were at the rear of the aircraft, some airlines go further with reduced baggage allowance and no meal.
The price was half the normal, not the highest economy fare. Quite a comfortable journey on a wide body with good seat pitch, decent catering and IFE compared to being sardine canned into a narrow body and bringing my own sandwiches.
The cabin was full both ways but I wonder what the yield was like if most people had bought the cheap tickets, bearing in mind the cost base of the airline was at legacy level.