The ending to Zamyatin's work is one where there is difficulty in finding hope.  D-503 has become a being where there is no hope for change.  The idea of "reason must prevail" is one whereby the individual's voice for freedom and change has become subjugated by the totalizing force of political society.  "We" has won, and the result is that D-503 is gone.  At the same time, O-90 has escaped in order to have her child in primitive world of freedom.  The construction of reality is one whereby freedom is something elusive, requiring individuals to risk everything in order to have it.  O-90 has to escape in order to be a human being and raise her child as one.  While there might be hope here, I think that Zamyatin is smart enough to avoid such a expected hope.  The fact that O-90 has lived her life without freedom for so long begs the question as to how easily she will be able to transform her life now that it is with her.  The lessons that must be taught to the child must first be taught to her in this new condition, one that is vastly different from the life she has led.  In this ending, there is a shroud of mystery as to how things will progress and whether there will be redemption for human beings.