I understand what your saying, however the bottom line is that BHX now has to try and fill a larger number of higher price seats that is my point
No, on United all the seats in economy/economy plus are sold as standard economy seats... Once a passenger has bought an economy class ticket, they have the option to pay more for extra leg-room as an add-on (United frequent flyer program members get the add-on for free). This add-on allows the passenger to select a seat towards the front of the cabin in the section called "economy plus" which has the extra leg room - if a passenger doesn't buy the add-on, the extra leg-room seats are blocked to them. If no one buys the extra leg room, these seats still get sold as normal economy seats and the lucky ones who book last minute may well get the extra leg room for free.
The net result is that economy class tickets are no more expensive - it's just that United gets some extra revenue from up-selling extra leg-room seats, otherwise known as "economy plus". The new United has made the decision that the extra revenue generated from up-selling extra leg-room seats compensates for the loss of six seats per aircraft.
Andy