If you really want to have a good (i.e. international quality) F product, you have to maintain its integrity and take upgrades out of the equation.
If you can drink the milk for free, why would buy the cow?
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".