It's nothing to do with "developing a major airport" if there's no hub capacity. Why are no major legacies at Gatwick today when it had great facilities and surface access? Why was Norman Fosters new airport, "London's Third Airport" built at great expense to LHR airlines by BAA eventually gifted to Ryanair for next to nothing?
Build a big airport all you like, if it's in the wrong place it's Mirabel.
Anyone who advocates Gatwick as the solution fails to see how the 21st century aviation MARKET actually behaves.