The following chart is based on a quite bold interpretation of the data available. By using the A2A (K+D) as a 'usage' metric and combining that with the individual top speeds at sea level, I tried to determine if the players are faster fighters these days. The result:
(Image removed from quote.)
Analysis:
The Spitfire is a connecting plane. It connects the turny world with the fast plane set. When lots of Spits are around, their speed is fast enough to force planes into turn fights, and Spit pilots are not afraid to engage in Turn fights. With the increase in turn fighting, there is good results to be had with a more turny bird (hurri, Ki-84, N1ki, Zeke, P-40, 109F, etc. But when the Spit population declines, the rest turny set is too slow to catch the Stangs, Jugs, Dora, La7 croud so players abandon the turny planes altogether and jump in Faster rides in an attempt to force action.
The Theory:
1) I think move to faster planes has been caused by the decline in spit fire use, which happened for its own reason. That reason is the "easy mode" reputation/Mocking in combination with increased player experience levels. As players progress they move away from the Spit into something more challenging. I think the experience level of the average player is gowing over the past few years because there are less new players to the game. IThese more experinced players begin to look for new challenges, such as the achievement system, GV-ing etc.
2) Also a contributing factor is the large rise in GV-ing. This has cause a shift in the planes set to more bombers and heavy fighters for bombing GVs. The American Plane set of F4Us, Pony's. Lightnings, Jugs, and P-38s are ideal for bombing GVs. They are also Faster and harder to catch so you need to bring your K4, Dora, La7 to kill them.
3) The increased GV action seems to coincide with more Base-Taking fights, than Furball Fights. If you're taking a base you bring Attack/Fighters like Pony's, F4Us Typhies, etc..and Bombers. Requiring faster planes catch diving, bomb dropping fighters.