Yeah, I'd say finish the last game. Whether it be a 5200 or 2600 version. I'd love to see it as a finished cartridge.
If Atari is being boneheaded about it keep trying to convince then that the completion of the Airworld game would be in their best interests. They may want to profit off of it themselves perhaps with a Swordquest plug and play or something like that, but they would need all four games for something like that to happen.
It's in the fans, and Atari's, best interests for Airworld to be finished. Keep trying with them.