Okay, from what I gather, you can play from playlists that have all the games' maps combined. Wouldn't that look bad for 343 if they let you go into matchmaking and play a map with Halo 1's dated graphics in one game, then the next game comes up and it's a Halo 4 map with pretty much new-ish graphics?
And why can't the original Halo 2 have new graphics? The campaign gets them.