The decline in the number of F class a/c is a reflection of the long world-wide economic downturn. The airlines don't have F for free upgrades or frequent flyer folks; they are for folks who are willing to pay $15,000 one way from SFO
etc. The global recession hit every sector including airlines.
will have some F class a/c. They have a few 767's, a ton of sUA 777's and the 744's. We will see the 744's exit the fleet over the next few years. The 788's won't have F. The 789's and presumably the 359's might have F. If the global economy takes off again, they will add more F as demand dictates.
The advent of Y+ and the decline of paying F passengers signal a shift. UA
will continue keeping capacity in-line with demand both in terms of flights and which classes are flown on each flight.