We'll, I guess it had to end when (almost) all of the original characters died. Bokurano was always about the shock factor, originality, interesting dialogue, adolescence, human emotions, psychology, and philosophical and ethical questions. In that sense, this is a masterpiece.
"I wonder, what existence designed this whole thing?"
"There are many people who say there's a God somewhere up there, but God is nothing more than a formula. Just a part of some law of physics that we have yet to understand."
Evolution prevails! :D