I... can't help but disagree with this. At the end of the story, Mylo could simply "go on to be trained", he doesn't need to be either a blackrunner or die by the end, inconclusions that imply the future are perfectly acceptable in media. Otherwise, things are too rushed, like everything has to happen NOW.
I can't really speak on whether one of the main three is going to die, but I would say that it's most likely Byte, if so. Alex or Loken dying would not be a valid conclusion to the story, even in sacrifice.
And lastly, they couldn't build an advanced society. Even in a truly happy ending, the world collapsed in the Cascade 500 years prior, so everyone would just be continuing out their lives in the ruins of the world, slowly trying to make things better (albeit without the blackzones around).