Traditionally with airlines, the front of the plane is where the money is made. And, traditionally, where US airlines have gone wrong, is that they have given much of the front of the plane away too cheaply. The key is to reward people who buy F, J or high Y fares (which now UA does through giving them more miles but, unless they are flying internationally in no other way) and then to incentivize those who have a propensity to buy quite a few of those fares even more. UA does this by mileage multipliers and all the other paraphernalia of GS, 1K etc.

It would be madness to sell all inventory in advance at a discount. You get far more for short notice full fare tickets and you know full well that you have means of selling on any distressed inventory - GPUs, RPUs, miles + copay, ToDs etc.