Originally Posted by entropy

Well, you kinda just put the hit the nail on the head wrt CO's way of giving away domestic F for Y/B fares. There is a big difference on an international flight where you charge $1000 for Y, $3-4k for Z (not upgradable), 8k for D+ and 10k for F. they don't give comp upgrades to F, you have to pay SWU or Cash+Miles, which have value. If you can motivate someone to spend 8k for D rather than 4k for Z with a SWU, that's "worth it".