Ok, I will be blunt. There is a cycle seen repeatedly in Aces High and most of us fit that mold.
Personally, I don't like the treadmill but the longer the game can keep the interest of subscribers, the bigger the online population and revenue for HTC. Even if it causes them to stay 2-3 months longer it would be worth the investment.
I agree with your list of the stages. I'm somewhere between the last two, especially with the low number of planes I see in the air when I log in.
There's really only 2 ways I could see implementing a treadmill. Grinding for planes (which would likely cause a current player mass exodus) or grinding for cosmetics. Personally I think it would rub the vast majority of longtime players wrong, although there aren't THAT many of them still around and it might bring in more than they'd lose. Historical skins be damned, they want their personalized purple planes.
The flipside to that coin is the kind of gamer who'd come in and want to get all the things they can grind for, once they've gotten all the things they either sit around and demand new content to grind for, or they leave. At the rate HTC turns out new content, they'd leave.
Wiley.