I think there's still plenty of story to be had surrounding the Halos. These things are big superweapons that can wipe out a galaxy. Surely there's some drama and conflict to be had there on a galactic scale... And if the Forerunners do remain in fiction, I would be upset if there's no further mystery or unknowns that come into play.

I think Halo 4's story, while having no Halos in game, is just part of what will be leading the series back into the Halos' re-emergence in some manner. The core story in the Halo universe, the "now", if you will (while many stories take place before and/or after) surrounds the chief and ~2552. The Halos are not gone. So I think as long as his timeline/era is still central to the haloverse and the Halos still exist, I think Halo remains an apt name.