But 40 is an average, so if they limited capacity to 40 the average would go down ie if loads over three days are 20, 40 and 60 your average is 40. If it is 20, 40 and 40 because there are no more seats the average is 33.3.
And if you put economy on board, you lose your USP as an 'exclusive' airline and start complicating your systems.