I'm reading the chart like this;...
11/2001 is when the AWIII crowd migrated over to AHI, AH got a big boost from that.... and the game held a steady 4000 all the way through 3/2011, 10 years.
The rest is peaks and valleys. As I read it 4000 was the core high number, follow that over and War Thunder and IL-2 releases, which are a different breed. Quite a few games have come and gone since AH started. steady 1000 from beginning to present.
The drop of big numbers started during the great recession of 08. There was already a distinct downward pattern before AHIII and Steam was released. AHIII certainly stopped the hard spiral, and leveled out. Unfortunately IMO it was after most left, they didn't get to see it.
From what I read on IL-2 boards, it's not doing all that well, according to posts in their forums "great graphics, but this and that..". On steam they are offering 30%-85% discounts during winter.
AW-AHIII is a very strong dedicated base for 35+ years. Not sure I know any other game that can say that. We also have players that have aged playing AW through AHIII. Many core players are passing away.
It's not the game,... IMO it's the niche genre.