I'm not sure why people think ground units are inevitable in a dogfighting game.
I agree with you that it's not inevitable, and I also agree with you that X-Wing, as we know it, is a simple dogfighting game.
I do think that if they want to make it more than that, that they have several avenues for doing so. Adding a ground-game element or a capital-ship scale element as a separate game is a way in order to make it more than what it is, if FFG wants to do that.
I would certainly want them to do that, rather than continue to flog this horse with endless dogfighting expansions, finally getting everything out of that image with all the weird and wonky TIE fighters. I could see a ground game being complementary to our game by introducing landing ships such as Sentinel-class landers and Theta-class dropships as something that needs to get past the screen of dog-fighting X-Wings in order to bring troops and walkers to the ground game.
I could see X-Wing being a game that exists between the capital-ships game (from where the landers/dropships are launched) and the ground game (where they disgorge their units).
That said, while I think it would be awesome, I think it's a long-shot bet that people would want to play that en masse. I don't get the sense that most people want to play integrated games. I think that - like you - they want short and sweet.