With respect to Frodo and Scotty, many people complained that the introduction of ground vehicles would "ruin" the game, with many calls of "It's a flight sim, stupid" hurled around. There are purists who still gripe about gv'ers constantly ruining their fun. The fact is, the introduction of gv's added another dimension that many have come to love about it. It was done in a skillful way that, rather than ruin the pure pilot's fun, actually adds a dimension to the airwar that was lacking before. Given HiTech's expressed desires to add submarines and ground troops (albeit in a somewhat limited fashion), it should be clear his long-term vision for AH is to make it accessable and enjoyable to a wider range of interests than just sim pilots; nonetheless, his vision is constrained by his commitment to maintain the airwar as the principle focus.
HiTech, I know you don't want to introduce the ground war in half measures, hence your comments on the resources you'll need to "do it right." However, I wonder how difficult it would be to allow players to spawn a trooper using the current non-articulated pilot model (in a different uniform, of course) and with the option to select from a limited range of weapons (riffle, grease gun, panzerschek, satchle charge, for example). This seems like a relatively easy and quick way to playtest the introduction of troops into the arena.
Fighting your way through an underground bunker (ala "Doom") is all very well, but allows no interaction with the other combat elements. Some would say that's how it should be. That interaction is what would separate the game from your average run of the mill FPS. Without that interaction, I can't see the addition of bunker-style FPS play as being a draw to outside players, those more interested in ground combat vs. A2A. Just my two cents worth.