I see lots of flights F1 or F0 more than five days from departure, and based on my own purchases, I think the decline is largely due to advance sales of F-cabin seats as mentioned by Kacee. On my most commonly flown route, a flight that is 3+ hours long, P is often available for $60-80 more one-way than coach (regardless of class available). If it is available for that price I always purchase it as there are very few last minute upgrades on that particular route, which I'm keeping top secret because I don't want you to all start buying up the cheap F-seats.

Unlike last year, I have gotten a couple on-board upgrades this year, due to either no-shows, which are fairly common on routes like ORD-IAH, DCA-IAH, and LGA-IAH or the fact that the TODs seem to jump in price when it gets down to 1 or 2 seats left. I have seen with traveling companies that an upgrade was available for $79 or $109 when there were three seats in F available, but then it was $569 when it was down to the last seat. Not too many people will pay $569 for an upgrade on a domestic flight, even on DCA-IAH.

IAH-MCO is a very reliable upgrade for me. And when purchasing only that segment the price for P is much higher than economy, which is probably why so many seats are left available for free upgrades. It may not be true if flying say OKC-IAH-MCO, I haven't checked upgrade prices, although the coach prices are routinely cheaper for the connection. (darn airline pricing!) Of course, it is likely not a Premier-heavy route on most days. But TPA, particularly around holidays, often has P for a very small premium over coach. Depending on my destination sometimes TPA is a decent option for me.

Last year: upgrade something like 98/100. This year: missed quite a few.