I would rather have a 3D background with no framerate lag than a 2D background with no framerate lag. Just because something isn't the worst it could possibly doesn't mean there's no room for improvement - especially when it's been done better for years.
Good point. I'm not impressed with Certain Affinity (especially their small BTB maps and extremely small Forge map terrains) but this majestic map pack feels really good, probably because people have come to expect just mediocre maps from them and these are slightly above that. Also they look beautiful so that doesn't hurt. The Majestic maps are better than a lot of the maps on disc too so they are pretty solid and that's why I like them. But you make a good point... they don't feel like true classics that anyone will want to remake in the future. None of the Halo 4 maps are... Haven comes the closest. Maybe with the 3 maps we have left we will get one...but doubtful. I'm sure the Castle maps will at least be as good as the Majestic ones though.
My point is that it's a tradeoff, something demonstrated by, well, Halo 4 at large. I'm damn sure which side I prefer to take a hit on, and I'm just thankful we actually have 3 fun maps that don't suffer from crippling framerate for the sake of these skybox bells and whistles. Halo 4 at large demonstrates that this simply isn't an option. It's been done better for years in games with worse graphics. The Xbox hasn't gotten any better in this time, but what's being asked of it has. Even Reach was pushing the hardware enough to bring out some pretty bad framerate issues, and Halo 4 took that to a whole new level. The fact that Halo 3 could do what you're asking is basically meaningless at this stage.