I wrote a paper on a similar topic a few months ago. In revenue management strategy, one variable which is considered is the "exclusivity value." The general idea is that empty seats make full seats more valuable. Therefore, most airlines set targets not just for seats filled, but seats left empty as well. Availability of upgrades (whether paid or by miles) is dependent on whether the load target is met, not whether seats are open.
So, for a hypothetical, if 9W was targeting a 50% load in F, and 60% of the seats are sold, they probably wouldn't be willing to sell an upgrade, but they might be willing to sell a full fare ticket. Over the past few years, it has seemed like 9W sets ridiculously low targets based on upgrade and award availability - however, only the people working at the revenue management department can say for sure what they are thinking (or if they are thinking at all).
Other airlines follow different strategies - UA, for example, targets 100% occupancy in all cabins as their revenue management strategy targets lower yields.