I agree, not going out on loan didn't seem to do Iwobi any harm either. I think Wilshere benefited massively from his loan, so did Song. Difference is though they were both 6 month loan deals to Premier League teams rather than multiple season long loan deals to League One sides.
I think Crowley, if he is to make it, would benefit from impressing in the reserves and cup games enough to attract a Premier League team, rather than go for a season to Oxford United. He'll need to do really well there to make any sort of impression.