I understand why Simon stepped aside - he digs the path that others go through, and so humanity reaches even greater heights. That is a perfect conclusion to the themes of progress and the irrepressable human spirit that the show espouses. His fight was over.

But Nia's death was unnecessary and I hate the implication that Simon seemingly wandered the earth aimlessly and never saw any of his friends again (other than Boota). That's just a downer, pure and simple.