Rise of Flight isn't exactly a popular game either, it's still getting updates. I bought the Iron Cross Edition and it's more than enough to keep me entertained ($30).
Rise of Flight also doesn't match your description of pay once, get full access, nor does it have significant ongoing maintenance costs.
1) It is not pay once. It is pay for the base game, pay for additional airplanes as they are released.
2) It is not an MMO and hence does not have the ongoing costs of server maintenance and internet traffic.
3) As a single player game you would continue to enjoy your copy even if the production/publisher went away. If HTC goes under AH is gone. Sure, you can play on your FE, but the fun aspect of the game is gone.
Just look at any of the new titles that came out within the last 5 years. They're all miles ahead graphics engine wise.
You're aware that most engines won't be suitable due to their short view distance that that using another company's engine is expensive, yes?