I don't know what's the situation with respect to the big cities in England but in France a sensible forecast can be that the big cities (Paris, Lyon, Marseille, Toulouse, Bordeaux, Grenoble, Nante, Nice, Toulon,...) will end up with a team in the Elite although not present yet. What we see is that the Rugby scene is quite dynamic and removing the Relegation/Promotion would stall this.
The situation may be different in England and Rugby may be considered to be the sport of the medium cities which cannot afford a good Football club and that there's no wil to popularize it in the desert areas. It seems to me that the GP want to reproduce the Rugby League situation which is well developped around the M62 and struggle to get out of this area.