How far is blacktown from Penrith? They struggle to get fans to a ground near by, will they attract anyone if they have to travel?
A Blacktown stadium, like a Liverpool stadium, IMO will have the same issues as the Olympic Park venues currently have; without any kind of supporting precinct around it, after the honeymoon period it will struggle to draw crowds on a regular basis. The most successful stadiums in Australia are all close to their respective cities' CBDs, within walking distance of established, well-policed entertainment districts where people and their families can relax in a safe environment before and after the game and within easy reach of public transport to all other parts of the city. Parramatta satisfies these criteria much better for the region than either of the above. A light rail link to Parramatta and/or Westmead station would also be a useful addition.
Clubs that are as dependent on their local area support as Penrith, or Cronulla, or Manly, and who won't countenance broadening this by raising their profile elsewhere in Sydney and moving to centralised grounds should be ready to be asked what place they have in a national competition if the NRL administration is competent.