Bungie and Microsoft should milk the Halo series and keep on bringing out new games. If Halo games keep selling millions of copies - it would make sense to continue the Halo series into the future. Final Fantasy, GTA series, Mario series, etc all have been milked to death - because they sell lots of copies and make huge profits for the companies.
Halo 3 has sold over 10 million copies and keeps on selling. Halo 3:ODST will sell between 5 to 6 million copies and it is set between Halo 2 and Halo 3 games. Imagine what sales numbers Halo: Reach and possibly Halo 4 would make if they are released in the future.
IMO XBox = Halo.