Years ago, there was a similar column in American Way magazine by whomever was in charge at the time (Crandall or Carty - I don't remember) which also pointed out that business travelers should be happy that the airline didn't sell out all the seats at low advance purchase fares, since that would make it impossible for last-minute travelers to walk up and say "I want to buy a ticket right now for the next flight out."
Implication being, of course, that the airline could have easily sold out all the seats on every flight if it priced and populated the various fare buckets to do that. Of course, holding empty perishable seats that might not be sold at all means that they are necessarily gonna be somewhat expensive for that walkup pax. They don't really mind the high prices, the column pointed out, because they tended to be business travelers for whom the trip was very important - so high fares were ok.
This is all well and good except for one factor: why not shrink the fleet size? Suddenly, instead of having a 200 seat 757 flying that route, a 150 seat A320 is. I'll even say it costs another penny per seat mile to fly that A320. Let's see how this impacts revenue:
-757: 200 x .10 x 1500 =$30k
-80% LF makes it 160 seats, each one paying $375 r/t
-Sell 100 of the seats for an average of $200 r/t
-Must now sell remaining 60 seats for $667 r/t
-A320: 150 x .11 x 1500 = $24,750
-80% LF makes it 120 seats, each one paying $413 r/t
-Sell 60 of the seats for an average of $200 r/t
-Must now sell remaining 60 seats for $625 r/t
That makes the tickets approx. 6% cheaper. Go to an A319 with 125 seats and it becomes:
-125 x .11 x 1500 = $20,625
-80% LF makes it 100 seats, each one paying $413 r/t
-Sell 40 of the seats for an average of $200 r/t
-Must now sell remaining 60 seats for $554 r/t
Which makes the tickets about 17% cheaper than using a 757.
Now, this does not take into effect other items, such as the fact that when you lower the amount of leisure seats, you can raise the ticket price on them, or that when you lower the price of a last minute ticket, you will stimulate demand, both of which will have postiive effects as well.