We have had the heights of happiness from Berserk, we need despair for the ending. As far as I had gotten (vol 30) the story was in "stoic" mode, shit was happening but it wasn't despair yet.
It's also true that the ending I came up with is, to my mind, the only way the ending could be satisfying. Guts has to die, inevitably, because he's lost everything of value except his hatred. Any further character relationship convolutions would simply weaken the series imo.