I dunno, I think theres a reasonable possibility that the Chief dies... They're clearly announced that this is the end of the his 'story arc'.
If the Chief dies to save humanity, he'll be the biggest hero ever in the Halo universe, if he lives.... then theres nothing stopping the story from continuing....
Heroism is more than just fighting and winning... if a hero chooses to make the ultimate sacrifice to save us all, then that hero is immortalized forever...
Bungie probably wants to keep making videogames beyond Halo, they've had a good run with Halo, but I'm sure that they don't want to get pigeonholed into doing ONLY Halo... they have to make a clean break... this might be the only way they can do it and go on to make more amazing franchises.