Halo Reach is simultaneously the last and first Halo game. It takes place before Halo:Combat Evolved on the planet Reach, and will be the last Halo game Bungie Studios will work on. That being said, I doubt Microsoft will retire the line at all.
Now the fun part about Reach, at least for me is wondering how tragic they plan to make it. The planet Reach is a huge loss for humanity as the Covenant attack, so we're going back to the beginning and we're losing, though if you played the rest of the series, you know how it ends. I'm really hoping there will be no zombalien Flood, but I wouldn't be surprised considering how much that can be tied in with Spartans.
But we are on path with trailers that are pretty tragic.