The question is if that feature is something we would LIKE to implement. I don´t think Omni´s being fighting with the submodel LOD´s only to see his work split up into multiple missions. I don´t see how "fly to the death star -> loading,briefing -> fly in the trench" is supposed to be immersive? Even if it meant(and in this case I don´t think it necessarily means) lower detail, I´d still choose the seamless experience before the two mission approach.
As for limiting the players movement to the trench? I think that´s something that fits Rogue Squadron perfectly. It´s just so... consoleish.
I think the games most of us draw inspiration from are X-wing, Tie-Fighter, XWA.. Games that feel more like Star Wars starfighter simulators than shooters in a Star Wars setting. Part of the simulator feeling is freedom. For example, the freedom to pull out of the trench when you feel the tactical situation demands it.
Besides, as far as I know, we don´t need to do these things for better detail.