I'd be interested in seeing whether the stats have changed (and I assume they have) with respect to the breakdown of F passengers who have paid for F seats (either full fare F, or a discounted F fare like a YUP) compared to F passengers who purchased Y seats and used miles or upgrades to get into F. I think the airlines, who used to be loath to discount their "premium" F product, have gotten much smarter and realized that they can use their F cabin to generate significant incremental revenue by selling F seats, albeit at reduced prices, rather than giving them away for free.

By the way, it is my suspicion that there are passengers sitting in Y who, for little or no extra cost, particularly when you factor in the cost of the 500-mile upgrades that many of them would have been willing to use to upgrade had the upgrade been available, could have purchased discounted F seats. But I think a lot of these people, and/or their travel agents, don't know enough to look for YUP and other similar fares when booking flights.