[milo]

I'm guessing the 2D movement was a DESIGN consideration

Yes, of course it was. As you rightly point out, adding 3D movement to a 3D environment is not a great technical hurdle. However, just providing the player with some kind of pitch control would not be sufficient. Just off the top of my head, the game would also need:

• New AI algorithms for NPC ships to take advantage of 3D
• New weapon and shield rules to account for elevation as well as azimuth
• New camera system to help player see targets above and below his ship
• 6DOF physics model for all ships
  

All that would result in a huge change of emphasis in the way Starfury plays. The game would become less goal-oriented, and more process-oriented. It would also become more difficult, especially for people who have a hard time solving spacial puzzles.