You are projecting your own narrow gameplay preferences on everyone else. What if many or even most weren't actually looking for 'better furballs'? How do you actually know why people left? Or better, why AH just didn't recruit enough new players, because players do leave games, even great and fun ones, all the time. What is your data base for a whopping '100%' knowledge?
Interestingly enough, we DID have such places for better furballs, the early EW & MW arenas and WWI. After the split about everyone was in EW & MW arenas first. And yet they left to return to LW. WWI was nothing but a close range furball for a month. Then players just went back to LW, never to be seen again for the most part.
If everybody just wanted the best and quickest furballs, why did they leve those places?
On the other hand, AH had all that, the hordes (even more extreme ones than today), the landgrabbing, the utter dominance of a very few 'endgame' fighters. Yet AH's number kept growing for years.
I think your view is much biased and very simplistic. It's as if I would claim "100%, the numbers are down because the central strats are gone" (which is actually my personal pet peeve).
I can tell you why I am thinking of leaving.
#1: I'm an Aussie and when I log on there are usually 20 players on. There is usually a number of teams of GV base takers one manning or two manning bases ever few minutes. There is usually no one to fight.
#2 I love fast hard hitting planes, Mossie, Tiffie. My favourite plane is the Typhoon. I don't fly it because it is so outmatched by a few planes that are very common. The La7 and now Yak3 being the most deadly.
# I feel almost forced to fly planes I don't like to fly (Spit, LA, Yak, F4U) to compete.
Whether I'm an example of why the average player who starts and quits is quitting is up for debate but these are some things that might be fixed pretty easily:
Planes
Why are most people going for the 5 or 6 specific planes they are?
Can something be done to reduce the number of these planes so more planes are represented in game?
Will perking these planes work?
Can something else be done other than perking? Carrot rather than stick?
Is the lack of diversity even the reason people are not staying?
Can something be done about AH reality vs WW2 reality (scaling of performance AH vs RL)?
Time Zones
Why are there some many GV base takers in downtime rather than in planes?
Is it because of the nerfed GV icons? Towns too easy to take down with just GV's and no air support?
Is the lack of players on non-prime time hours related to a lack of people in planes because they're in GV's taking bases? Is the lack of players in non-prime time hours reducing player numbers in non-prime time hours? (does a guy subscribe, find no one to fight in planes, so unsubscribes?).
Can something be done to alleviate that? Etc Etc.
Can bases be buffed in non-prime time hours? Etc ETc.