No company in there right mind would forsake a money maker like Halo. Its the flagship of the Xbox. From what I can remember (I was young) Xbox was worthless. Who would get an Xbox? It had no real games that made it different or desirable from Gamecube or the PS2. Then Halo came out, and the thing was selling like hotcakes. Of course they are going to bring it back, and Bungie is an idiot for jumping off the ship.
First, for the money, success and fame. Yes, I understand that there is more to life then that. But Halo put Bungie on the radar of the general population. Marathon and ONI? Do you know how hard pressed you'd be to find people that know about those games, let alone played them before Halo came out? You'd need search and rescue.
The second is that they washed their hands and turned away from what they "loved" and left it in the hands of Microsoft who is going to rape the shit out of that series until it becomes another Mario or Zelda thing.
I wouldn't give up on Halo until after it comes out. Lets see what they can do with it.