Long story short, after High Road to Revenge was shipped, the team started to develop yet another Crimson Skies sequel before Microsoft decided to scrap the entire project and focus on other things. Later, through an interesting turn of circumstances, the Crimson Skies franchise was purchased by the ex-Microsoft employee founded company Smith & Tinker where it currently resides. A company who admittiedly loves the Crimson Skies franchise, but is currently busy with the Capcom published Dark Void.
All said, Crimson Skies is in loving hands with Smith & Tinker, but a sequel isn't high priority right now. Though, they do leave the door open to developing more airplane funnery, admitting that they have "a lot of Crimson Skies fans around here, and if the stars aligned, it'd be something we'd definitely be willing to explore again." Our fingers are crossed.