I am not familiar with the proposal you mention, but live in one of the towns mentioned above (I have sons/daughters at both the HS and youth level), and do not agree at all that holdbacks are a factor for these programs.
In reality, players who become "holdbacks" actually tend to leave these public school programs after freshman/sophomore year and repeat at a private school (either day or boarding), so are not even part of the public HS teams after that.
While there may be 1-2 exceptions for New Canaan HS or Darien HS over the course of any 5 year period, those 1-2 players are hardly the "difference makers" for either program's success.