A good point was made that a model of neurogenesis must also incorporate neuronal death besides neuronal birth (since hippocampus and the brain as a whole have physical constraints, you can't keep growing your network infinitely :). That's why any model of neurogenesis must incorporate interplay between birth and death of new (and old) neurons; that's was the main idea of the paper I mentioned in an earlier post (this year ICLR submission https://openreview.net/forum?id=HyecJGP5ge) Note that just adding nodes to networks was proposed before, eg. the classical work on cascade correlations.