Fewer possible ways to play the game means less players.
Removal of NOE, improved radar, increased town sizes, improved AAA, increased number of hangars, ... and fewer players (compared to the early days). This draws a lot more action to the PvE element of the game, leaving less room for PvP air-to-air action. While the changes are probably intended to improve that, they don't convert an NOE field grabber into a 1-vs-1 dogfighter, and force an extended focus on ground targets to play the team objective (*).
The game isn't the one single best game for 1-vs-1 fights. There is competition that didn't exist 20 years ago. Dogfights (1-vs-1 or furballs without a purpose) can be had elsewhere, for a fixed one-time purchase, with better looking graphics, too. It doesn't work as the worlds best flight simulator either, others are providing a much more detailed simulation.
The monthly payment is only worth it for the persistent world with its different tactical options and strategic elements. Those are what make it unique. Many changes haven't influenced these topics in a positive way I think.
None of the advertising, not the website, steam page, emphasize this aspect. New people join while expecting certain things, based on the material consumed before. Then they notice that the game doesn't deliver on those points, at least not any better than alternatives, and at a monthly payment, and they leave.
An "easy" way to increase the retention rate is to pull in the right kind of players, that are looking for what the game has to offer. "Great dogfights here!" draws in players that do expect 15$/month-better-than-elsewhere dogfights, and quickly notice that they won't get what they were looking for. "Complex strategic game with planes!" might draw in less players (not necessarily), but a lot more might stay (**).
(*) The probability to encounter an enemy tank preventing capture is much higher than encountering an enemy fighter that is looking for the goon, mostly because goons don't work anymore - so the proper action is to always take off heavy.
(**) When I first read about this game (actually, WB, but that's the same more or less), it was on a squad page that described the concept of the war, with planes as the means to fight the war. Would a description of doom-with-planes have drawn me in? Unlikely. With more alternatives available, even less today.